diff --git a/dune-project b/dune-project
index 864325aa31..a2389a4bbc 100644
--- a/dune-project
+++ b/dune-project
@@ -24,13 +24,13 @@ The client-side code is compiled to JS using Ocsigen Js_of_ocaml or to Wasm usin
   ocamlfind
   ppx_deriving
   (ppxlib (>= 0.15.0))
-  (js_of_ocaml-compiler (>= 3.6.0))
-  (js_of_ocaml (>= 3.6.0))
-  (js_of_ocaml-lwt (>= 3.6.0))
+  (js_of_ocaml-compiler (>= 5.5.0))
+  (js_of_ocaml (>= 5.5.0))
+  (js_of_ocaml-lwt (>= 5.5.0))
   (js_of_ocaml-ocamlbuild :build)
-  (js_of_ocaml-ppx (>= 3.6.0))
-  (js_of_ocaml-ppx_deriving_json (>= 3.6.0))
-  (js_of_ocaml-tyxml (>= 3.6.0))
+  (js_of_ocaml-ppx (>= 5.5.0))
+  (js_of_ocaml-ppx_deriving_json (>= 5.5.0))
+  (js_of_ocaml-tyxml (>= 5.5.0))
   lwt_log
   (lwt_ppx (>= 1.2.3))
   (tyxml (and (>= 4.6.0) (< 4.7.0)))
diff --git a/eliom.opam b/eliom.opam
index ddf6028a36..52419c16e2 100644
--- a/eliom.opam
+++ b/eliom.opam
@@ -22,13 +22,13 @@ depends: [
   "ocamlfind"
   "ppx_deriving"
   "ppxlib" {>= "0.15.0"}
-  "js_of_ocaml-compiler" {>= "3.6.0"}
-  "js_of_ocaml" {>= "3.6.0"}
-  "js_of_ocaml-lwt" {>= "3.6.0"}
+  "js_of_ocaml-compiler" {>= "5.5.0"}
+  "js_of_ocaml" {>= "5.5.0"}
+  "js_of_ocaml-lwt" {>= "5.5.0"}
   "js_of_ocaml-ocamlbuild" {build}
-  "js_of_ocaml-ppx" {>= "3.6.0"}
-  "js_of_ocaml-ppx_deriving_json" {>= "3.6.0"}
-  "js_of_ocaml-tyxml" {>= "3.6.0"}
+  "js_of_ocaml-ppx" {>= "5.5.0"}
+  "js_of_ocaml-ppx_deriving_json" {>= "5.5.0"}
+  "js_of_ocaml-tyxml" {>= "5.5.0"}
   "lwt_log"
   "lwt_ppx" {>= "1.2.3"}
   "tyxml" {>= "4.6.0" & < "4.7.0"}
diff --git a/src/lib/client/eliom_client.js b/src/lib/client/eliom_client.js
index ca73d37ded..be0465361a 100644
--- a/src/lib/client/eliom_client.js
+++ b/src/lib/client/eliom_client.js
@@ -3,7 +3,7 @@
 //Provides: caml_unwrap_value_from_string
 //Requires: caml_failwith, caml_marshal_constants
 //Requires: caml_int64_float_of_bits, caml_int64_of_bytes, caml_new_string
-//Requires: caml_jsbytes_of_string
+//Requires: caml_jsbytes_of_string, caml_callback
 var caml_unwrap_value_from_string = function (){
   function StringReader (s, i) { this.s = caml_jsbytes_of_string(s); this.i = i; }
   StringReader.prototype = {
@@ -222,7 +222,7 @@ var caml_unwrap_value_from_string = function (){
         if (v[0] === 0 && size >= 2 &&
             v[size] instanceof Array && v[size].length == 3 &&
             v[size][2] === intern_obj_table[2] /*unwrap_mark*/) {
-          var unwrapped_v = apply_unwrapper(v[size], v);
+          var unwrapped_v = caml_callback(apply_unwrapper, [v[size], v]);
           if (unwrapped_v === 0) {
             // No unwrapper is registered, so replace the unwrap
             // marker v[size] by a late_unwrap marker
diff --git a/src/lib/client/eliom_client.wat b/src/lib/client/eliom_client.wat
new file mode 100644
index 0000000000..561629cad1
--- /dev/null
+++ b/src/lib/client/eliom_client.wat
@@ -0,0 +1,656 @@
+(module
+   (import "env" "caml_failwith" (func $caml_failwith (param (ref eq))))
+   (import "env" "caml_init_custom_operations"
+      (func $caml_init_custom_operations))
+   (import "env" "caml_find_custom_operations"
+      (func $caml_find_custom_operations
+         (param (ref $string)) (result (ref null $custom_operations))))
+   (import "env" "caml_callback_2"
+      (func $caml_callback_2
+         (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))))
+
+   (type $block (array (mut (ref eq))))
+   (type $string (array (mut i8)))
+   (type $float (struct (field f64)))
+   (type $float_array (array (mut f64)))
+   (type $js (struct (field anyref)))
+
+   (type $compare
+      (func (param (ref eq)) (param (ref eq)) (param i32) (result i32)))
+   (type $hash
+      (func (param (ref eq)) (result i32)))
+   (type $fixed_length (struct (field $bsize_32 i32) (field $bsize_64 i32)))
+   (type $serialize
+      (func (param (ref eq)) (param (ref eq)) (result i32) (result i32)))
+   (type $deserialize (func (param (ref eq)) (result (ref eq)) (result i32)))
+   (type $dup (func (param (ref eq)) (result (ref eq))))
+   (type $custom_operations
+      (struct
+         (field $id (ref $string))
+         (field $compare (ref null $compare))
+         (field $compare_ext (ref null $compare))
+         (field $hash (ref null $hash))
+         (field $fixed_length (ref null $fixed_length))
+         (field $serialize (ref null $serialize))
+         (field $deserialize (ref null $deserialize))
+         (field $dup (ref null $dup))))
+   (type $custom (sub (struct (field (ref $custom_operations)))))
+
+   (data $bad_length "unwrap_value: bad length")
+
+   (func (export "caml_unwrap_value_from_string")
+      (param $unwrapper (ref eq))
+      (param $vstr (ref eq)) (param $vofs (ref eq)) (result (ref eq))
+      (local $str (ref $string))
+      (local $ofs i32)
+      (local $s (ref $intern_state))
+      (local $h (ref $marshal_header))
+      (local.set $str (ref.cast (ref $string) (local.get $vstr)))
+      (local.set $ofs (i31.get_u (ref.cast (ref i31) (local.get $vofs))))
+      (local.set $s
+         (call $get_intern_state (local.get $str) (local.get $ofs)))
+      (local.set $h (call $parse_header (local.get $s)))
+      (if (i32.gt_s
+             (i32.add (local.get $ofs)
+                (i32.add (struct.get $marshal_header $data_len (local.get $h))
+                   (i32.const 20)))
+             (array.len (local.get $str)))
+         (then
+            (call $caml_failwith
+               (array.new_data $string $bad_length
+                  (i32.const 0) (i32.const 24)))))
+      (return_call $intern_rec
+         (local.get $unwrapper) (local.get $s) (local.get $h)))
+
+   (global $Intext_magic_number_small i32 (i32.const 0x8495A6BE))
+   (global $Intext_magic_number_big i32 (i32.const 0x8495A6BF))
+
+   (global $PREFIX_SMALL_BLOCK i32 (i32.const 0x80))
+   (global $PREFIX_SMALL_INT i32 (i32.const 0x40))
+   (global $PREFIX_SMALL_STRING i32 (i32.const 0x20))
+   (global $CODE_INT8 i32 (i32.const 0x00))
+   (global $CODE_INT16 i32 (i32.const 0x01))
+   (global $CODE_INT32 i32 (i32.const 0x02))
+   (global $CODE_INT64 i32 (i32.const 0x03))
+   (global $CODE_SHARED8 i32 (i32.const 0x04))
+   (global $CODE_SHARED16 i32 (i32.const 0x05))
+   (global $CODE_SHARED32 i32 (i32.const 0x06))
+   (global $CODE_BLOCK32 i32 (i32.const 0x08))
+   (global $CODE_BLOCK64 i32 (i32.const 0x13))
+   (global $CODE_STRING8 i32 (i32.const 0x09))
+   (global $CODE_STRING32 i32 (i32.const 0x0A))
+   (global $CODE_DOUBLE_BIG i32 (i32.const 0x0B))
+   (global $CODE_DOUBLE_LITTLE i32 (i32.const 0x0C))
+   (global $CODE_DOUBLE_ARRAY8_BIG i32 (i32.const 0x0D))
+   (global $CODE_DOUBLE_ARRAY8_LITTLE i32 (i32.const 0x0E))
+   (global $CODE_DOUBLE_ARRAY32_BIG i32 (i32.const 0x0F))
+   (global $CODE_DOUBLE_ARRAY32_LITTLE i32 (i32.const 0x07))
+   (global $CODE_CODEPOINTER i32 (i32.const 0x10))
+   (global $CODE_INFIXPOINTER i32 (i32.const 0x11))
+   (global $CODE_CUSTOM i32 (i32.const 0x12))
+   (global $CODE_CUSTOM_LEN i32 (i32.const 0x18))
+   (global $CODE_CUSTOM_FIXED i32 (i32.const 0x19))
+
+   ;; Keep in sync with marshal.wat
+   (type $intern_state
+      (struct
+         (field $src (ref $string))
+         (field $pos (mut i32))
+         (field $obj_table (mut (ref null $block)))
+         (field $obj_counter (mut i32))))
+
+   (func $get_intern_state
+      (param $src (ref $string)) (param $pos i32) (result (ref $intern_state))
+      (struct.new $intern_state
+         (local.get $src) (local.get $pos) (ref.null $block)
+         (i32.const 0)))
+
+   (func $read8u (param $s (ref $intern_state)) (result i32)
+      (local $pos i32) (local $res i32)
+      (local.set $pos (struct.get $intern_state $pos (local.get $s)))
+      (local.set $res
+         (array.get_u $string
+            (struct.get $intern_state $src (local.get $s))
+            (local.get $pos)))
+      (struct.set $intern_state $pos (local.get $s)
+         (i32.add (local.get $pos) (i32.const 1)))
+      (local.get $res))
+
+   (func $read8s (param $s (ref $intern_state)) (result i32)
+      (local $pos i32) (local $res i32)
+      (local.set $pos (struct.get $intern_state $pos (local.get $s)))
+      (local.set $res
+         (array.get_s $string
+            (struct.get $intern_state $src (local.get $s))
+            (local.get $pos)))
+      (struct.set $intern_state $pos (local.get $s)
+         (i32.add (local.get $pos) (i32.const 1)))
+      (local.get $res))
+
+   (func $read16u (param $s (ref $intern_state)) (result i32)
+      (local $src (ref $string)) (local $pos i32) (local $res i32)
+      (local.set $src (struct.get $intern_state $src (local.get $s)))
+      (local.set $pos (struct.get $intern_state $pos (local.get $s)))
+      (local.set $res
+         (i32.or
+            (i32.shl
+               (array.get_u $string (local.get $src) (local.get $pos))
+               (i32.const 8))
+            (array.get_u $string (local.get $src)
+               (i32.add (local.get $pos) (i32.const 1)))))
+      (struct.set $intern_state $pos (local.get $s)
+         (i32.add (local.get $pos) (i32.const 2)))
+      (local.get $res))
+
+   (func $read16s (param $s (ref $intern_state)) (result i32)
+      (local $src (ref $string)) (local $pos i32) (local $res i32)
+      (local.set $src (struct.get $intern_state $src (local.get $s)))
+      (local.set $pos (struct.get $intern_state $pos (local.get $s)))
+      (local.set $res
+         (i32.or
+            (i32.shl
+               (array.get_s $string (local.get $src) (local.get $pos))
+               (i32.const 8))
+            (array.get_u $string (local.get $src)
+               (i32.add (local.get $pos) (i32.const 1)))))
+      (struct.set $intern_state $pos (local.get $s)
+         (i32.add (local.get $pos) (i32.const 2)))
+      (local.get $res))
+
+   (func $read32 (param $s (ref $intern_state)) (result i32)
+      (local $src (ref $string)) (local $pos i32) (local $res i32)
+      (local.set $src (struct.get $intern_state $src (local.get $s)))
+      (local.set $pos (struct.get $intern_state $pos (local.get $s)))
+      (local.set $res
+         (i32.or
+            (i32.or
+               (i32.shl
+                  (array.get_u $string (local.get $src) (local.get $pos))
+                  (i32.const 24))
+               (i32.shl
+                  (array.get_u $string (local.get $src)
+                     (i32.add (local.get $pos) (i32.const 1)))
+                  (i32.const 16)))
+            (i32.or
+               (i32.shl
+                  (array.get_u $string (local.get $src)
+                     (i32.add (local.get $pos) (i32.const 2)))
+                  (i32.const 8))
+               (array.get_u $string (local.get $src)
+                  (i32.add (local.get $pos) (i32.const 3))))))
+      (struct.set $intern_state $pos (local.get $s)
+         (i32.add (local.get $pos) (i32.const 4)))
+      (local.get $res))
+
+   (func $readblock (param $s (ref $intern_state)) (param $str (ref $string))
+      (local $len i32) (local $pos i32)
+      (local.set $len (array.len (local.get $str)))
+      (local.set $pos (struct.get $intern_state $pos (local.get $s)))
+      (array.copy $string $string
+         (local.get $str) (i32.const 0)
+         (struct.get $intern_state $src (local.get $s)) (local.get $pos)
+         (local.get $len))
+      (struct.set $intern_state $pos (local.get $s)
+         (i32.add (local.get $pos) (local.get $len))))
+
+   (func $readstr (param $s (ref $intern_state)) (result (ref $string))
+      (local $len i32) (local $pos i32) (local $res (ref $string))
+      (local $src (ref $string))
+      (local.set $src (struct.get $intern_state $src (local.get $s)))
+      (local.set $pos (struct.get $intern_state $pos (local.get $s)))
+      (loop $loop
+         (if (array.get_u $string (local.get $src)
+                (i32.add (local.get $pos) (local.get $len)))
+            (then
+               (local.set $len (i32.add (local.get $len) (i32.const 1)))
+               (br $loop))))
+      (local.set $res (array.new $string (i32.const 0) (local.get $len)))
+      (array.copy $string $string
+         (local.get $res) (i32.const 0)
+         (local.get $src) (local.get $pos)
+         (local.get $len))
+      (struct.set $intern_state $pos (local.get $s)
+         (i32.add (local.get $pos) (i32.add (local.get $len) (i32.const 1))))
+      (local.get $res))
+
+   (func $readfloat
+      (param $s (ref $intern_state)) (param $code i32) (result f64)
+      (local $src (ref $string)) (local $pos i32) (local $res i32)
+      (local $d i64)
+      (local $i i32)
+      (local $v (ref eq))
+      (local.set $src (struct.get $intern_state $src (local.get $s)))
+      (local.set $pos (struct.get $intern_state $pos (local.get $s)))
+      (struct.set $intern_state $pos (local.get $s)
+         (i32.add (local.get $pos) (i32.const 8)))
+      (if (i32.eq (local.get $code) (global.get $CODE_DOUBLE_BIG))
+         (then
+            (loop $loop
+               (local.set $d
+                  (i64.or
+                     (i64.shl (local.get $d) (i64.const 8))
+                     (i64.extend_i32_u
+                        (array.get_u $string (local.get $src)
+                          (i32.add (local.get $pos) (local.get $i))))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br_if $loop (i32.lt_u (local.get $i) (i32.const 8)))))
+         (else
+            (loop $loop
+               (local.set $d
+                  (i64.rotr
+                     (i64.or (local.get $d)
+                        (i64.extend_i32_u
+                           (array.get_u $string (local.get $src)
+                             (i32.add (local.get $pos) (local.get $i)))))
+                     (i64.const 8)))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br_if $loop (i32.lt_u (local.get $i) (i32.const 8))))))
+      (f64.reinterpret_i64 (local.get $d)))
+
+   (func $readfloats
+      (param $s (ref $intern_state)) (param $code i32) (param $len i32)
+      (result (ref eq))
+      (local $dest (ref $float_array))
+      (local $i i32)
+      (local.set $code
+         (select (global.get $CODE_DOUBLE_BIG) (global.get $CODE_DOUBLE_LITTLE)
+            (i32.or
+               (i32.eq (local.get $code) (global.get $CODE_DOUBLE_ARRAY8_BIG))
+               (i32.eq (local.get $code)
+                 (global.get $CODE_DOUBLE_ARRAY32_BIG)))))
+      (local.set $dest (array.new $float_array (f64.const 0) (local.get $len)))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (array.set $float_array (local.get $dest) (local.get $i)
+                  (call $readfloat (local.get $s) (local.get $code)))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (local.get $dest))
+
+   (func $register_object (param $s (ref $intern_state)) (param $v (ref eq))
+      (local $p i32)
+      (local.set $p (struct.get $intern_state $obj_counter (local.get $s)))
+      (array.set $block
+         (struct.get $intern_state $obj_table (local.get $s))
+         (local.get $p) (local.get $v))
+      (struct.set $intern_state $obj_counter (local.get $s)
+         (i32.add (local.get $p) (i32.const 1))))
+
+   (data $unknown_custom "unwrap_value: unknown custom block identifier")
+   (data $expected_size "unwrap_value: expected a fixed-size custom block")
+   (data $incorrect_size
+      "unwrap_value: incorrect length of serialized custom block")
+
+   (func $intern_custom
+      (param $s (ref $intern_state)) (param $code i32) (result (ref eq))
+      (local $ops (ref $custom_operations))
+      (local $expected_size i32)
+      (local $r (tuple (ref eq) i32))
+      (block $unknown
+         (local.set $ops
+            (br_on_null $unknown
+               (call
+                  $caml_find_custom_operations
+                  (call $readstr
+                     (local.get $s)))))
+         (block $no_length
+            (if (i32.eq (local.get $code) (global.get $CODE_CUSTOM_FIXED))
+               (then
+                  (local.set $expected_size
+                     (struct.get $fixed_length $bsize_32
+                        (br_on_null $no_length
+                           (struct.get $custom_operations $fixed_length
+                              (local.get $ops))))))
+            (else
+               (if (i32.eq (local.get $code) (global.get $CODE_CUSTOM_LEN))
+                  (then
+                     (local.set $expected_size (call $read32 (local.get $s)))
+                     (struct.set $intern_state $pos (local.get $s)
+                        (i32.add (struct.get $intern_state $pos (local.get $s))
+                           (i32.const 8)))))))
+            (local.set $r
+               (call_ref $deserialize (local.get $s)
+                  (struct.get $custom_operations $deserialize (local.get $ops))))
+            (if (i32.and
+                  (i32.ne (tuple.extract 2 1 (local.get $r))
+                     (local.get $expected_size))
+                  (i32.ne (local.get $code) (global.get $CODE_CUSTOM)))
+               (then
+                  (call $caml_failwith
+                     (array.new_data $string $incorrect_size
+                        (i32.const 0) (i32.const 57)))))
+            (return (tuple.extract 2 0 (local.get $r))))
+         (call $caml_failwith
+            (array.new_data $string $expected_size
+               (i32.const 0) (i32.const 48))))
+      (call $caml_failwith
+         (array.new_data $string $unknown_custom
+            (i32.const 0) (i32.const 45)))
+      (ref.i31 (i32.const 0)))
+
+   (data $integer_too_large "unwrap_value: integer too large")
+   (data $code_pointer "unwrap_value: code pointer")
+   (data $ill_formed "unwrap_value: ill-formed message")
+   (data $incorrect_value "unwrap_value: incorrect value")
+
+   (type $stack_item
+      (struct
+         (field $blk (ref $block))
+         (field $pos (mut i32))
+         (field $ofs i32)
+         (field $next (ref null $stack_item))))
+
+   (func $intern_rec
+      (param $unwrapper (ref eq))
+      (param $s (ref $intern_state)) (param $h (ref $marshal_header))
+      (result (ref eq))
+      (local $late_unwrap_mark (ref $block))
+      (local $res (ref $block)) (local $dest (ref $block))
+      (local $sp (ref $stack_item))
+      (local $code i32)
+      (local $header i32) (local $tag i32) (local $size i32)
+      (local $len i32) (local $pos i32) (local $pos' i32) (local $ofs i32)
+      (local $b (ref $block))
+      (local $str (ref $string))
+      (local $v (ref eq)) (local $v' (ref eq))
+      (call $caml_init_custom_operations)
+      (local.set $late_unwrap_mark (array.new_fixed $block 0))
+      (local.set $res (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
+      (local.set $sp
+         (struct.new $stack_item
+            (local.get $res) (i32.const 0) (i32.const -1)
+            (ref.null $stack_item)))
+      (local.set $size (struct.get $marshal_header $num_objects (local.get $h)))
+      (struct.set $intern_state $obj_table (local.get $s)
+         (array.new $block (ref.i31 (i32.const 0)) (local.get $size)))
+      (local.set $v (ref.i31 (i32.const 0))) ;; keep validator happy
+      (block $exit
+       (loop $loop
+        (block $done
+         (block $read_block
+          (block $read_string
+           (block $read_double_array
+            (block $read_shared
+             (local.set $code (call $read8u (local.get $s)))
+             (if (i32.ge_u (local.get $code) (global.get $PREFIX_SMALL_INT))
+                (then
+                   (if (i32.ge_u (local.get $code) (global.get $PREFIX_SMALL_BLOCK))
+                      (then
+                         ;; Small block
+                         (local.set $tag
+                            (i32.and (local.get $code) (i32.const 0xF)))
+                         (local.set $size
+                            (i32.and (i32.shr_u (local.get $code) (i32.const 4))
+                               (i32.const 0x7)))
+                         (br $read_block))
+                      (else
+                         ;; Small int
+                         (local.set $v
+                            (ref.i31
+                               (i32.and (local.get $code) (i32.const 0x3F))))
+                         (br $done))))
+                (else
+                   (if (i32.ge_u (local.get $code)
+                         (global.get $PREFIX_SMALL_STRING))
+                      (then
+                         (local.set $len
+                            (i32.and (local.get $code) (i32.const 0x1F)))
+                         (br $read_string))
+                      (else
+                         (block $INT8
+                          (block $INT16
+                           (block $INT32
+                            (block $INT64
+                             (block $SHARED8
+                              (block $SHARED16
+                               (block $SHARED32
+                                (block $BLOCK32
+                                 (block $STRING8
+                                  (block $STRING32
+                                   (block $DOUBLE
+                                    (block $DOUBLE_ARRAY8
+                                     (block $DOUBLE_ARRAY32
+                                      (block $CODEPOINTER
+                                       (block $CUSTOM
+                                        (block $default
+                                         (br_table $INT8 $INT16 $INT32 $INT64
+                                            $SHARED8 $SHARED16 $SHARED32
+                                            $DOUBLE_ARRAY32 $BLOCK32 $STRING8
+                                            $STRING32 $DOUBLE $DOUBLE
+                                            $DOUBLE_ARRAY8 $DOUBLE_ARRAY8
+                                            $DOUBLE_ARRAY32 $CODEPOINTER
+                                            $CODEPOINTER $CUSTOM $default
+                                            $default $default $default $default
+                                            $CUSTOM $CUSTOM $default
+                                            (local.get $code)))
+                                        ;; default
+                                        (call $caml_failwith
+                                           (array.new_data $string $ill_formed
+                                              (i32.const 0) (i32.const 32)))
+                                        (br $done))
+                                       ;; CUSTOM
+                                       (local.set $v
+                                          (call $intern_custom (local.get $s)
+                                             (local.get $code)))
+                                       (call $register_object (local.get $s)
+                                          (local.get $v))
+                                       (br $done))
+                                      ;; CODEPOINTER
+                                      (call $caml_failwith
+                                        (array.new_data $string $code_pointer
+                                           (i32.const 0) (i32.const 26)))
+                                      (br $done))
+                                     ;; DOUBLE_ARRAY32
+                                     (local.set $len
+                                        (call $read32 (local.get $s)))
+                                     (br $read_double_array))
+                                    ;; DOUBLE_ARRAY8
+                                    (local.set $len
+                                       (call $read8u (local.get $s)))
+                                    (br $read_double_array))
+                                   ;; DOUBLE
+                                   (local.set $v
+                                      (struct.new $float
+                                         (call $readfloat
+                                            (local.get $s) (local.get $code))))
+                                   (call $register_object
+                                      (local.get $s) (local.get $v))
+                                   (br $done))
+                                  ;; STRING32
+                                  (local.set $len (call $read32 (local.get $s)))
+                                  (br $read_string))
+                                 ;; STRING8
+                                 (local.set $len (call $read8u (local.get $s)))
+                                 (br $read_string))
+                                ;; BLOCK32
+                                (local.set $header (call $read32 (local.get $s)))
+                                (local.set $tag
+                                   (i32.and (local.get $header)
+                                      (i32.const 0xFF)))
+                                (local.set $size
+                                   (i32.shr_u (local.get $header)
+                                      (i32.const 10)))
+                                (br $read_block))
+                               ;; SHARED32
+                               (local.set $ofs (call $read32 (local.get $s)))
+                               (br $read_shared))
+                              ;; SHARED16
+                              (local.set $ofs (call $read16u (local.get $s)))
+                              (br $read_shared))
+                             ;; SHARED8
+                             (local.set $ofs (call $read8u (local.get $s)))
+                             (br $read_shared))
+                            ;; INT64
+                            (call $caml_failwith
+                               (array.new_data $string $integer_too_large
+                                  (i32.const 0) (i32.const 31)))
+                            (br $done))
+                           ;; INT32
+                           (local.set $v (ref.i31 (call $read32 (local.get $s))))
+                           (br $done))
+                          ;; INT16
+                          (local.set $v (ref.i31 (call $read16s (local.get $s))))
+                          (br $done))
+                         ;; INT8
+                         (local.set $v (ref.i31 (call $read8s (local.get $s))))
+                         (br $done))
+                        ))))
+            ;; read_shared
+            (local.set $ofs
+               (i32.sub
+                  (struct.get $intern_state $obj_counter (local.get $s))
+                  (local.get $ofs)))
+            (local.set $v
+               (array.get $block
+                  (struct.get $intern_state $obj_table (local.get $s))
+                  (local.get $ofs)))
+            (br_if $done (i32.eqz (ref.test (ref $block) (local.get $v))))
+            (local.set $b (ref.cast (ref $block) (local.get $v)))
+            (local.set $len (array.len (local.get $b)))
+            (br_if $done (i32.lt_u (local.get $len) (i32.const 2)))
+            (local.set $v'
+               (array.get $block (local.get $b)
+                  (i32.sub (local.get $len) (i32.const 1))))
+            (br_if $done (i32.eqz (ref.test (ref $block) (local.get $v'))))
+            (local.set $b (ref.cast (ref $block) (local.get $v')))
+            (br_if $done (i32.ne (array.len (local.get $b)) (i32.const 4)))
+            (br_if $done
+               (i32.eqz
+                  (ref.eq (array.get $block (local.get $b) (i32.const 2))
+                     (local.get $late_unwrap_mark))))
+            (array.set $block (local.get $b) (i32.const 3)
+               (array.new_fixed $block 3 (ref.i31 (i32.const 0))
+                  (array.new_fixed $block 3 (ref.i31 (i32.const 0))
+                     (struct.get $stack_item $blk (local.get $sp))
+                     (ref.i31
+                        (struct.get $stack_item $pos (local.get $sp))))
+                  (array.get $block (local.get $b) (i32.const 3))))
+            (br $done))
+           ;; read_double_array
+           (local.set $v
+              (call $readfloats
+                 (local.get $s) (local.get $code) (local.get $len)))
+           (call $register_object (local.get $s) (local.get $v))
+           (br $done))
+          ;; read_string
+          (local.set $str (array.new $string (i32.const 0) (local.get $len)))
+          (call $readblock (local.get $s) (local.get $str))
+          (local.set $v (local.get $str))
+          (call $register_object (local.get $s) (local.get $v))
+          (br $done))
+         ;; read_block
+         (local.set $b
+            (array.new $block (ref.i31 (i32.const 0))
+               (i32.add (local.get $size) (i32.const 1))))
+         (array.set $block (local.get $b) (i32.const 0)
+            (ref.i31 (local.get $tag)))
+         (if (local.get $size)
+            (then
+               (local.set $sp
+                  (struct.new $stack_item
+                     (local.get $b) (i32.const 1)
+                     (struct.get $intern_state $obj_counter (local.get $s))
+                     (local.get $sp)))
+               (call $register_object (local.get $s) (local.get $b))
+               (br $loop)))
+         (local.set $v (local.get $b))
+         (br $done))
+        ;; done
+        (loop $assign
+           (local.set $dest (struct.get $stack_item $blk (local.get $sp)))
+           (local.set $pos (struct.get $stack_item $pos (local.get $sp)))
+           (array.set $block (local.get $dest) (local.get $pos) (local.get $v))
+           (local.set $pos' (i32.add (local.get $pos) (i32.const 1)))
+           (struct.set $stack_item $pos (local.get $sp) (local.get $pos'))
+           (local.set $len (array.len (local.get $dest)))
+           (br_if $loop (i32.ne (local.get $pos') (local.get $len)))
+           (local.set $v (local.get $dest))
+           (local.set $ofs (struct.get $stack_item $ofs (local.get $sp)))
+           (local.set $sp
+              (br_on_null $exit (struct.get $stack_item $next (local.get $sp))))
+           (br_if $assign (i32.lt_u (local.get $len) (i32.const 2)))
+           (br_if $assign
+              (i32.eqz
+                 (ref.eq (array.get $block (local.get $dest) (i32.const 0))
+                    (ref.i31 (i32.const 0)))))
+           (local.set $v'
+              (array.get $block (local.get $dest)
+                 (i32.sub (local.get $len) (i32.const 1))))
+           (br_if $assign (i32.eqz (ref.test (ref $block) (local.get $v'))))
+           (local.set $b (ref.cast (ref $block) (local.get $v')))
+           (br_if $assign (i32.ne (array.len (local.get $b)) (i32.const 3)))
+           (br_if $assign
+              (i32.eqz
+                 (ref.eq (array.get $block (local.get $b) (i32.const 2))
+                    (array.get $block
+                       (struct.get $intern_state $obj_table (local.get $s))
+                       (i32.const 1)))))
+           (local.set $v'
+              (call $caml_callback_2 (local.get $unwrapper)
+                 (local.get $b) (local.get $dest)))
+           (if (ref.test (ref $block) (local.get $v'))
+              (then
+                 (local.set $v
+                    (array.get $block (ref.cast (ref $block) (local.get $v'))
+                       (i32.const 1)))
+                  (array.set $block
+                     (struct.get $intern_state $obj_table (local.get $s))
+                     (local.get $ofs) (local.get $v)))
+              (else
+                 (array.set $block (local.get $dest)
+                    (i32.sub (local.get $len) (i32.const 1))
+                    (array.new_fixed $block 4 (ref.i31 (i32.const 0))
+                       (array.get $block (local.get $b) (i32.const 1))
+                       (local.get $late_unwrap_mark)
+                       (array.new_fixed $block 3 (ref.i31 (i32.const 0))
+                          (array.new_fixed $block 3 (ref.i31 (i32.const 0))
+                             (struct.get $stack_item $blk (local.get $sp))
+                             (ref.i31
+                                (struct.get $stack_item $pos (local.get $sp))))
+                          (ref.i31 (i32.const 0)))))))
+           (br $assign))))
+      (drop (block $incorrect_value (result (ref eq))
+         (local.set $b
+            (br_on_cast_fail $incorrect_value (ref eq) (ref $block)
+               (array.get $block (local.get $res) (i32.const 0))))
+         (if (i32.eq (array.len (local.get $b)) (i32.const 3))
+            (then (return (array.get $block (local.get $b) (i32.const 2)))))
+         (ref.i31 (i32.const 0))))
+      (call $caml_failwith
+         (array.new_data $string $incorrect_value (i32.const 0) (i32.const 29)))
+      (ref.i31 (i32.const 0)))
+
+   (data $too_large
+      "unwrap_value: object too large to be read back on a 32-bit platform")
+   (data $bad_object "unwrap_value: bad object")
+
+   (type $marshal_header
+      (struct
+         (field $data_len i32)
+         (field $num_objects i32)))
+
+   (func $parse_header
+      (param $s (ref $intern_state))
+      (result (ref $marshal_header))
+      (local $magic i32)
+      (local $data_len i32) (local $num_objects i32) (local $whsize i32)
+      (local.set $magic (call $read32 (local.get $s)))
+      (if (i32.eq (local.get $magic) (global.get $Intext_magic_number_big))
+         (then
+            (call $caml_failwith
+               (array.new_data $string $too_large
+                  (i32.const 0) (i32.const 67)))))
+      (if (i32.ne (local.get $magic) (global.get $Intext_magic_number_small))
+         (then
+            (call $caml_failwith
+               (array.new_data $string $bad_object
+                  (i32.const 0) (i32.const 24)))))
+      (local.set $data_len (call $read32 (local.get $s)))
+      (local.set $num_objects (call $read32 (local.get $s)))
+      (drop (call $read32 (local.get $s)))
+      (drop (call $read32 (local.get $s)))
+      (struct.new $marshal_header
+         (local.get $data_len)
+         (local.get $num_objects)))
+)
diff --git a/src/lib/client/eliommod_cookies.ml b/src/lib/client/eliommod_cookies.ml
index 15ac11441e..0e3607bddc 100644
--- a/src/lib/client/eliommod_cookies.ml
+++ b/src/lib/client/eliommod_cookies.ml
@@ -30,6 +30,109 @@ let cookie_tables :
   =
   Jstable.create ()
 
+module Map (Ord : sig
+    type key [@@deriving json]
+
+    val compare : key -> key -> int
+  end) =
+struct
+  type 'a t =
+    | Empty
+    | Node of {l : 'a t; v : Ord.key; d : 'a; r : 'a t; h : int}
+  [@@deriving json]
+
+  let height = function Empty -> 0 | Node {h; _} -> h
+
+  let create l x d r =
+    let hl = height l and hr = height r in
+    Node {l; v = x; d; r; h = (if hl >= hr then hl + 1 else hr + 1)}
+
+  let bal l x d r =
+    let hl = height l and hr = height r in
+    if hl > hr + 2
+    then
+      match l with
+      | Empty -> invalid_arg "Map.bal"
+      | Node {l = ll; v = lv; d = ld; r = lr; _} -> (
+          if height ll >= height lr
+          then create ll lv ld (create lr x d r)
+          else
+            match lr with
+            | Empty -> invalid_arg "Map.bal"
+            | Node {l = lrl; v = lrv; d = lrd; r = lrr; _} ->
+                create (create ll lv ld lrl) lrv lrd (create lrr x d r))
+    else if hr > hl + 2
+    then
+      match r with
+      | Empty -> invalid_arg "Map.bal"
+      | Node {l = rl; v = rv; d = rd; r = rr; _} -> (
+          if height rr >= height rl
+          then create (create l x d rl) rv rd rr
+          else
+            match rl with
+            | Empty -> invalid_arg "Map.bal"
+            | Node {l = rll; v = rlv; d = rld; r = rlr; _} ->
+                create (create l x d rll) rlv rld (create rlr rv rd rr))
+    else Node {l; v = x; d; r; h = (if hl >= hr then hl + 1 else hr + 1)}
+
+  let rec add x data = function
+    | Empty -> Node {l = Empty; v = x; d = data; r = Empty; h = 1}
+    | Node {l; v; d; r; h} as m ->
+        let c = Ord.compare x v in
+        if c = 0
+        then if d == data then m else Node {l; v = x; d = data; r; h}
+        else if c < 0
+        then
+          let ll = add x data l in
+          if l == ll then m else bal ll v d r
+        else
+          let rr = add x data r in
+          if r == rr then m else bal l v d rr
+
+  let rec fold f m accu =
+    match m with
+    | Empty -> accu
+    | Node {l; v; d; r; _} -> fold f r (f v d (fold f l accu))
+
+  let empty = Empty
+end
+
+[@@@warning "-39"]
+
+module Map_path = Map (struct
+    type key = string list [@@deriving json]
+
+    let compare = compare
+  end)
+
+module Map_inner = Map (struct
+    type key = string [@@deriving json]
+
+    let compare = compare
+  end)
+
+[@@@warning "+39"]
+
+let json_cookies =
+  [%json: (float option * string * bool) Map_inner.t Map_path.t]
+
+let extern_cookies c =
+  Ocsigen_cookie_map.Map_path.fold
+    (fun path inner m ->
+       Map_path.add path
+         (Ocsigen_cookie_map.Map_inner.fold Map_inner.add inner Map_inner.empty)
+         m)
+    c Map_path.empty
+
+let intern_cookies c =
+  Map_path.fold
+    (fun path inner m ->
+       Ocsigen_cookie_map.Map_path.add path
+         (Map_inner.fold Ocsigen_cookie_map.Map_inner.add inner
+            Ocsigen_cookie_map.Map_inner.empty)
+         m)
+    c Ocsigen_cookie_map.Map_path.empty
+
 (** [in_local_storage] implements cookie substitutes for iOS WKWebView *)
 let get_table ?(in_local_storage = false) = function
   | None -> Ocsigen_cookie_map.Map_path.empty
@@ -44,7 +147,8 @@ let get_table ?(in_local_storage = false) = function
              Js.Opt.case
                st ## (getItem host)
                (fun () -> Ocsigen_cookie_map.Map_path.empty)
-               (fun v -> Json.unsafe_input v))
+               (fun v ->
+                  intern_cookies (of_json ~typ:json_cookies (Js.to_string v))))
       else
         Js.Optdef.get
           (Jstable.find cookie_tables (Js.string host))
@@ -61,12 +165,15 @@ let set_table ?(in_local_storage = false) host t =
         Js.Optdef.case
           Dom_html.window##.localStorage
           (fun () -> ())
-          (fun st -> st ## (setItem host (Json.output t)))
+          (fun st ->
+             st
+             ## (setItem host
+                   (Js.string (to_json ~typ:json_cookies (extern_cookies t)))))
       else Jstable.add cookie_tables (Js.string host) t
 
 let now () =
   let date = new%js Js.date_now in
-  date##getTime /. 1000.
+  Js.to_float date##getTime /. 1000.
 
 (** [in_local_storage] implements cookie substitutes for iOS WKWebView *)
 let update_cookie_table ?(in_local_storage = false) host cookies =
diff --git a/src/lib/client/eliommod_dom.ml b/src/lib/client/eliommod_dom.ml
index 5113701f0b..22c4b67cb1 100644
--- a/src/lib/client/eliommod_dom.ml
+++ b/src/lib/client/eliommod_dom.ml
@@ -83,7 +83,7 @@ let fast_ancessor (elt1 : #Dom.node Js.t) (elt2 : #Dom.node Js.t) =
 
 let slow_ancessor (elt1 : #Dom.node Js.t) (elt2 : #Dom.node Js.t) =
   let rec check_parent n =
-    if n == (elt1 :> Dom.node Js.t)
+    if Js.strict_equals n (elt1 :> Dom.node Js.t)
     then true
     else
       match Js.Opt.to_option n##.parentNode with
@@ -162,20 +162,20 @@ let slow_has_classes (node : Dom_html.element Js.t) =
   let found_attrib = ref false in
   for i = 0 to classes##.length - 1 do
     found_call_service :=
-      Js.array_get classes i
-      == Js.def (Js.string Eliom_runtime.RawXML.ce_call_service_class)
+      Js.Optdef.strict_equals (Js.array_get classes i)
+        (Js.def (Js.string Eliom_runtime.RawXML.ce_call_service_class))
       || !found_call_service;
     found_process_node :=
-      Js.array_get classes i
-      == Js.def (Js.string Eliom_runtime.RawXML.process_node_class)
+      Js.Optdef.strict_equals (Js.array_get classes i)
+        (Js.def (Js.string Eliom_runtime.RawXML.process_node_class))
       || !found_process_node;
     found_closure :=
-      Js.array_get classes i
-      == Js.def (Js.string Eliom_runtime.RawXML.ce_registered_closure_class)
+      Js.Optdef.strict_equals (Js.array_get classes i)
+        (Js.def (Js.string Eliom_runtime.RawXML.ce_registered_closure_class))
       || !found_closure;
     found_attrib :=
-      Js.array_get classes i
-      == Js.def (Js.string Eliom_runtime.RawXML.ce_registered_attr_class)
+      Js.Optdef.strict_equals (Js.array_get classes i)
+        (Js.def (Js.string Eliom_runtime.RawXML.ce_registered_attr_class))
       || !found_attrib
   done;
   !found_call_service, !found_process_node, !found_closure, !found_attrib
@@ -185,8 +185,8 @@ let slow_has_request_class (node : Dom_html.element Js.t) =
   let found_request_node = ref false in
   for i = 0 to classes##.length - 1 do
     found_request_node :=
-      Js.array_get classes i
-      == Js.def (Js.string Eliom_runtime.RawXML.request_node_class)
+      Js.Optdef.strict_equals (Js.array_get classes i)
+        (Js.def (Js.string Eliom_runtime.RawXML.request_node_class))
       || !found_request_node
   done;
   !found_request_node
@@ -683,8 +683,13 @@ let preload_css (doc : Dom_html.element Js.t) =
    Dom_html.document##body while on Firefox they are found on
    Dom_html.document##documentElement. *)
 
+[@@@warning "-39"]
+
 type position =
   {html_top : int; html_left : int; body_top : int; body_left : int}
+[@@deriving json]
+
+[@@@warning "+39"]
 
 let top_position = {html_top = 0; html_left = 0; body_top = 0; body_left = 0}
 
@@ -783,10 +788,11 @@ let onhashchange f =
   else
     let last_fragment = ref Dom_html.window##.location##.hash in
     let check () =
-      if !last_fragment != Dom_html.window##.location##.hash
+      if not (Js.equals !last_fragment Dom_html.window##.location##.hash)
       then (
         last_fragment := Dom_html.window##.location##.hash;
         f Dom_html.window##.location##.hash)
     in
     ignore
-      Dom_html.window ## (setInterval (Js.wrap_callback check) (0.2 *. 1000.))
+      Dom_html.window
+      ## (setInterval (Js.wrap_callback check) (Js.float (0.2 *. 1000.)))
diff --git a/src/lib/client/eliommod_dom.mli b/src/lib/client/eliommod_dom.mli
index 3eb542b763..5c849d137e 100644
--- a/src/lib/client/eliommod_dom.mli
+++ b/src/lib/client/eliommod_dom.mli
@@ -87,6 +87,7 @@ val iter_attrList :
 
 type position =
   {html_top : int; html_left : int; body_top : int; body_left : int}
+[@@deriving json]
 
 val top_position : position
 val getDocumentScroll : unit -> position
diff --git a/src/lib/eliom_client.client.ml b/src/lib/eliom_client.client.ml
index f6cf6d266a..518ade5c35 100644
--- a/src/lib/eliom_client.client.ml
+++ b/src/lib/eliom_client.client.ml
@@ -165,7 +165,7 @@ let get_element_cookies_info elt =
     (Js.Opt.map
        elt
        ## (getAttribute (Js.string Eliom_runtime.RawXML.ce_call_service_attrib))
-       (fun s -> of_json (Js.to_string s)))
+       (fun s -> of_json ~typ:[%json: bool * string list] (Js.to_string s)))
 
 let get_element_template elt =
   Js.Opt.to_option
@@ -581,25 +581,38 @@ let add_string_event_listener o e f capt : unit =
    behaviour in not required by the HTML5 specification (only
    suggested). *)
 
+[@@@warning "-39"]
+
 type state =
   { (* TODO store cookies_info in state... *)
     template : string option
   ; position : Eliommod_dom.position }
+[@@deriving json]
+
+[@@@warning "+39"]
 
 let random_int =
   if Js.Optdef.test Js.Unsafe.global##.crypto
      && Js.Optdef.test Js.Unsafe.global##.crypto##.getRandomValues
   then
     fun () ->
-    Typed_array.unsafe_get
+    let a =
       Js.Unsafe.global ##. crypto
-      ## (getRandomValues (new%js Typed_array.int32Array 1))
-      0
-  else fun () -> truncate (4294967296. *. Js.math##random)
+      ## (getRandomValues (new%js Typed_array.int16Array 2))
+    in
+    (Typed_array.unsafe_get a 0 lsl 16) lor Typed_array.unsafe_get a 1
+  else fun () -> truncate (4294967296. *. Js.to_float Js.math##random)
 
 let section_page = Lwt_log.Section.make "eliom:client:page"
 
+[@@@warning "-39"]
+
 type state_id = {session_id : int; state_index : int (* point in history *)}
+[@@deriving json]
+
+type saved_state = state_id * string [@@deriving json]
+
+[@@@warning "+39"]
 
 module Page_status_t = struct
   type t = Generating | Active | Cached | Dead
@@ -813,13 +826,14 @@ let get_state state_id : state =
           Lwt_log.raise_error_f ~section "sessionStorage not available")
        (fun s -> s ## (getItem (state_key state_id))))
     (fun () -> raise Not_found)
-    (fun s -> Json.unsafe_input s)
+    (fun s -> of_json ~typ:[%json: state] (Js.to_string s))
 
 let set_state i (v : state) =
   Js.Optdef.case
     Dom_html.window##.sessionStorage
     (fun () -> ())
-    (fun s -> s ## (setItem (state_key i) (Json.output v)))
+    (fun s ->
+       s ## (setItem (state_key i) (Js.string (to_json ~typ:[%json: state] v))))
 
 let update_state () =
   set_state !active_page.page_id
@@ -1035,10 +1049,17 @@ let init () =
     Js._false
   in
   Lwt_log.ign_debug ~section "Set load/onload events";
-  onload_handler :=
-    Some
-      (Dom.addEventListener Dom_html.window (Dom.Event.make "load")
-         (Dom.handler onload) Js._true);
+  if Dom_html.document##.readyState = Js.string "complete"
+  then
+    Lwt.async @@ fun () ->
+    let%lwt () = Js_of_ocaml_lwt.Lwt_js_events.request_animation_frame () in
+    let _ = onload () in
+    Lwt.return_unit
+  else
+    onload_handler :=
+      Some
+        (Dom.addEventListener Dom_html.window (Dom.Event.make "load")
+           (Dom.handler onload) Js._true);
   add_string_event_listener Dom_html.window "beforeunload" onbeforeunload_fun
     false;
   ignore
@@ -1363,7 +1384,9 @@ let change_url_string ~replace uri =
     then (
       Opt.iter stash_reload_function !reload_function;
       Dom_html.window##.history##replaceState
-        (Js.Opt.return (this_page.page_id, Js.string full_uri))
+        (Js.Opt.return
+           (Js.string
+              (to_json ~typ:[%json: saved_state] (this_page.page_id, full_uri))))
         (Js.string "")
         (if !Eliom_common.is_client_app
          then Js.null
@@ -1372,7 +1395,9 @@ let change_url_string ~replace uri =
       update_state ();
       Opt.iter stash_reload_function !reload_function;
       Dom_html.window##.history##pushState
-        (Js.Opt.return (this_page.page_id, Js.string full_uri))
+        (Js.Opt.return
+           (Js.string
+              (to_json ~typ:[%json: saved_state] (this_page.page_id, full_uri))))
         (Js.string "")
         (if !Eliom_common.is_client_app
          then Js.null
@@ -2164,7 +2189,10 @@ let () =
        Dom_html.window ##. history
        ## (replaceState
              (Js.Opt.return
-                (!active_page.page_id, Dom_html.window##.location##.href))
+                (Js.string
+                   (to_json ~typ:[%json: saved_state]
+                      ( !active_page.page_id
+                      , Js.to_string Dom_html.window##.location##.href ))))
              (Js.string "") Js.null);
        Lwt.return_unit);
     Dom_html.window##.onpopstate
@@ -2172,10 +2200,13 @@ let () =
       Lwt_log.ign_debug ~section:section_page "revisit_wrapper: onpopstate";
       Eliommod_dom.touch_base ();
       Js.Opt.case
-        ((Js.Unsafe.coerce event)##.state
-         : (state_id * Js.js_string Js.t) Js.opt)
+        ((Js.Unsafe.coerce event)##.state : _ Js.opt)
         (fun () -> () (* Ignore dummy popstate event fired by chromium. *))
-        (fun (state, full_uri) -> revisit_wrapper (Js.to_string full_uri) state);
+        (fun saved_state ->
+           let state, full_uri =
+             of_json ~typ:[%json: saved_state] (Js.to_string saved_state)
+           in
+           revisit_wrapper full_uri state);
       Js._false))
   else
     (* Without history API *)
diff --git a/src/lib/eliom_comet.client.ml b/src/lib/eliom_comet.client.ml
index a4d9cd601d..3acf50c92d 100644
--- a/src/lib/eliom_comet.client.ml
+++ b/src/lib/eliom_comet.client.ml
@@ -138,7 +138,7 @@ module Configuration = struct
       then
         match (get ()).time_between_request_unfocused, focused () with
         | Some ((a, b, c) :: l), Some start ->
-            let now = (new%js Js.date_now)##getTime in
+            let now = Js.to_float (new%js Js.date_now)##getTime in
             (* time from idle start *)
             let t =
               max 0. (((now -. start) *. 0.001) -. (get ()).time_after_unfocus)
@@ -305,7 +305,9 @@ end = struct
     in
     let suspend_activity () =
       if handler.hd_activity.focused = None
-      then handler.hd_activity.focused <- Some (new%js Js.date_now)##getTime
+      then
+        handler.hd_activity.focused <-
+          Some (Js.to_float (new%js Js.date_now)##getTime)
     in
     let visibility_change_callback () =
       if document_hidden () then suspend_activity () else resume_activity ()
@@ -322,7 +324,7 @@ end = struct
         if tbru = Some [0., 0., 0.] (* Always active *)
         then `Active
         else
-          let now = (new%js Js.date_now)##getTime in
+          let now = Js.to_float (new%js Js.date_now)##getTime in
           if now -. t
              < (Configuration.get ()).Configuration.time_after_unfocus *. 1000.
           then `Active
@@ -338,7 +340,7 @@ end = struct
       then
         hd.hd_activity.focused <-
           Some
-            ((new%js Js.date_now)##getTime
+            (Js.to_float (new%js Js.date_now)##getTime
             -. ((Configuration.get ()).Configuration.time_after_unfocus *. 1000.)
             ))
     else hd.hd_activity.focused <- None;
diff --git a/src/lib/eliom_common_base.shared.ml b/src/lib/eliom_common_base.shared.ml
index c52cce965a..5ecc5b9f8a 100644
--- a/src/lib/eliom_common_base.shared.ml
+++ b/src/lib/eliom_common_base.shared.ml
@@ -177,11 +177,16 @@ let nl_is_persistent n = n.[0] = 'p'
 
 (*****************************************************************************)
 
+[@@@warning "-39"]
+
 type client_process_info =
   { cpi_ssl : bool
   ; cpi_hostname : string
   ; cpi_server_port : int
-  ; cpi_original_full_path : Url.path }
+  ; cpi_original_full_path : string list }
+[@@deriving json]
+
+[@@@warning "+39"]
 
 type sess_info =
   { si_other_get_params : (string * string) list
diff --git a/src/lib/eliom_lib.client.ml b/src/lib/eliom_lib.client.ml
index 73e65094b3..c7a037f3db 100644
--- a/src/lib/eliom_lib.client.ml
+++ b/src/lib/eliom_lib.client.ml
@@ -146,19 +146,29 @@ end
 (* We do not use the deriving (un)marshaling even if typ is available
    because direct jsn (un)marshaling is very fast client side
 *)
-let to_json ?typ:_ s = Js.to_string (Json.output s)
-let of_json ?typ:_ v = Json.unsafe_input (Js.string v)
-
-(* to marshal data and put it in a form *)
-let encode_form_value x = to_json x
+let to_json ?typ s =
+  match Sys.backend_type with
+  | Other "js_of_ocaml" -> Js.to_string (Json.output s)
+  | _ -> (
+    match typ with
+    | Some typ -> Deriving_Json.to_string typ s
+    | None -> Js.to_string (Json.output s))
+
+let of_json ?typ v =
+  match Sys.backend_type with
+  | Other "js_of_ocaml" -> Json.unsafe_input (Js.string v)
+  | _ -> (
+    match typ with
+    | Some typ -> Deriving_Json.from_string typ v
+    | None -> assert false)
 
 (* Url.urlencode ~with_plus:true (Marshal.to_string x [])
     (* I encode the data because it seems that multipart does not
        like \0 character ... *)
 *)
-let encode_header_value x =
+let encode_header_value ~typ x =
   (* We remove end of lines *)
-  String.remove_eols (to_json x)
+  String.remove_eols (to_json ~typ x)
 
 let unmarshal_js var = Marshal.from_string (Js.to_bytestring var) 0
 
diff --git a/src/lib/eliom_lib.client.mli b/src/lib/eliom_lib.client.mli
index cbe71f3694..1446dc976f 100644
--- a/src/lib/eliom_lib.client.mli
+++ b/src/lib/eliom_lib.client.mli
@@ -38,8 +38,8 @@ include
 
 type file_info = File.file Js.t
 
-val to_json : ?typ:'a -> 'b -> string
-val of_json : ?typ:'a -> string -> 'b
+val to_json : ?typ:'a Deriving_Json.t -> 'a -> string
+val of_json : ?typ:'a Deriving_Json.t -> string -> 'a
 
 module Url : sig
   (** URL manipulation *)
@@ -126,9 +126,8 @@ val confirm : ('a, unit, string, bool) format4 -> 'a
 val debug_var : string -> 'a -> unit
 val trace : ('a, unit, string, unit) format4 -> 'a
 val lwt_ignore : ?message:string -> unit Lwt.t -> unit
-val encode_form_value : 'a -> string
 val unmarshal_js : Js.js_string Js.t -> 'a
-val encode_header_value : 'a -> string
+val encode_header_value : typ:'a Deriving_Json.t -> 'a -> string
 
 val make_cryptographic_safe_string : ?len:int -> unit -> string
 (** Return a base-64 encoded cryptographic safe string of the given length.
diff --git a/src/lib/eliom_request.client.ml b/src/lib/eliom_request.client.ml
index 6ff0a58b1d..c7583cd47e 100644
--- a/src/lib/eliom_request.client.ml
+++ b/src/lib/eliom_request.client.ml
@@ -180,7 +180,10 @@ let send ?with_credentials ?(expecting_process_page = false) ?cookies_info
     let headers =
       match cookies with
       | [] -> []
-      | _ -> [Eliom_common.tab_cookies_header_name, encode_header_value cookies]
+      | _ ->
+          [ ( Eliom_common.tab_cookies_header_name
+            , encode_header_value ~typ:[%json: (string * string) list] cookies )
+          ]
     in
     let headers =
       if Js.Optdef.test Js.Unsafe.global##.___eliom_use_cookie_substitutes_
@@ -191,7 +194,7 @@ let send ?with_credentials ?(expecting_process_page = false) ?cookies_info
             path
         in
         ( Eliom_common.cookie_substitutes_header_name
-        , encode_header_value cookies )
+        , encode_header_value ~typ:[%json: (string * string) list] cookies )
         :: headers
       else headers
     in
@@ -209,7 +212,9 @@ let send ?with_credentials ?(expecting_process_page = false) ?cookies_info
       match host with
       | Some host when host = Url.Current.host ->
           ( Eliom_common.tab_cpi_header_name
-          , encode_header_value (Eliom_process.get_info ()) )
+          , encode_header_value
+              ~typ:[%json: Eliom_common_base.client_process_info]
+              (Eliom_process.get_info ()) )
           :: headers
       | _ -> headers
     in
@@ -228,7 +233,8 @@ let send ?with_credentials ?(expecting_process_page = false) ?cookies_info
           else "application/xhtml+xml"
         in
         ("Accept", content_type)
-        :: (Eliom_common.expecting_process_page_name, encode_header_value true)
+        :: ( Eliom_common.expecting_process_page_name
+           , encode_header_value ~typ:[%json: bool] true )
         :: headers
       else headers
     in
diff --git a/src/lib/eliom_unwrap.client.ml b/src/lib/eliom_unwrap.client.ml
index 6a52a73e47..4bb93c6223 100644
--- a/src/lib/eliom_unwrap.client.ml
+++ b/src/lib/eliom_unwrap.client.ml
@@ -72,11 +72,12 @@ let apply_unwrapper unwrapper v =
 let late_unwrap_value old_value new_value =
   let old_value = Obj.repr old_value in
   List.iter
-    (fun {parent; field} -> Js.Unsafe.set parent field new_value)
+    (fun {parent; field} ->
+       Obj.set_field parent (field - 1) (Obj.repr new_value))
     (Obj.obj (Obj.field (Obj.field old_value (Obj.size old_value - 1)) 2))
 
 external raw_unmarshal_and_unwrap :
-   (unit, unwrapper -> _ -> _ option) Js.meth_callback
+   (unwrapper -> _ -> _ option)
   -> string
   -> int
   -> _
@@ -85,7 +86,7 @@ external raw_unmarshal_and_unwrap :
 let unwrap s i =
   if !Eliom_config.debug_timings
   then Firebug.console ## (time (Js.string "unwrap"));
-  let res = raw_unmarshal_and_unwrap (Js.wrap_callback apply_unwrapper) s i in
+  let res = raw_unmarshal_and_unwrap apply_unwrapper s i in
   if !Eliom_config.debug_timings
   then Firebug.console ## (timeEnd (Js.string "unwrap"));
   res