diff --git a/acandy.lua b/acandy.lua
index 3a16a36..bf10a7e 100644
--- a/acandy.lua
+++ b/acandy.lua
@@ -27,7 +27,8 @@ local VOID_ELEMS, HTML_ELEMS, RAW_TEXT_ELEMS = (function ()
 	local config = require('.elem_config')
 	return config.VOID_ELEMS, config.HTML_ELEMS, config.RAW_TEXT_ELEMS
 end)()
-
+local classes = require('.classes')
+local node_mts = classes.node_mts
 
 ---@class Symbol
 
@@ -61,47 +62,51 @@ end
 ---@type metatable
 local Fragment_mt
 
----Append the serialized string of the Fragment to `strs`.
----Use len to avoid calling `#strs` repeatedly. This improves performance by
+---Append the serialized string of the Fragment to `buff`.
+---Use len to avoid calling `#buff` repeatedly. This improves performance by
 ---~1/3.
----@param strs table
+---@param buff table
 ---@param frag table
----@param strs_len? integer number of `strs`, used to optimize performance.
+---@param buff_len? integer length of `buff`, used to optimize performance.
 ---@param no_encode? boolean true to prevent encoding strings, e.g. when in <script>.
-local function extend_strings_with_fragment(strs, frag, strs_len, no_encode)
+local function extend_str_buff_with_frag(buff, frag, buff_len, no_encode)
 	if #frag == 0 then return end
-	strs_len = strs_len or #strs
+	buff_len = buff_len or #buff
 
 	local function append_serialized(node)
 		local node_type = type(node)
-		if container_level_of(node) >= 1 then  -- Fragment
+		if container_level_of(node) >= 1 then  -- Fragment, list
 			for _, child_node in ipairs(node) do
 				append_serialized(child_node)
 			end
 		elseif node_type == 'function' then
 			append_serialized(node())
 		elseif node_type == 'string' then
-			strs_len = strs_len + 1
-			strs[strs_len] = no_encode and node or utils.html_encode(node)
+			buff_len = buff_len + 1
+			buff[buff_len] = no_encode and node or utils.html_encode(node)
 		else  -- others: Raw, Element, boolean, number
-			strs_len = strs_len + 1
-			strs[strs_len] = tostring(node)
+			local str = tostring(node)
+			if not (node_mts[getmt(node)] or no_encode) then
+				str = utils.html_encode(str)
+			end
+			buff_len = buff_len + 1
+			buff[buff_len] = str
 		end
 	end
 
 	append_serialized(frag)
 end
 
-Fragment_mt = {
+Fragment_mt = node_mts:register {
 	---Flat and concat the Fragment, returns string.
 	---@param self Fragment
 	---@return string
 	__tostring = function (self)
 		if #self == 0 then return '' end
 
-		local children = {}
-		extend_strings_with_fragment(children, self, 0)
-		return concat(children)
+		local buff = {}
+		extend_str_buff_with_frag(buff, self, 0)
+		return concat(buff)
 	end,
 	__index = {
 		concat = table.concat,
@@ -111,8 +116,7 @@ Fragment_mt = {
 		move = table.move,
 		remove = table.remove,
 		sort = table.sort,
-		---@diagnostic disable-next-line: deprecated
-		unpack = table.unpack or unpack,
+		unpack = table.unpack or unpack,  ---@diagnostic disable-line: deprecated
 	},
 	[KEY_LIST_LIKE] = true,
 }
@@ -172,29 +176,29 @@ end
 ---@class Breadcrumb
 
 
----@param strs string[]
+---@param buff string[]
 ---@param attr_map {[string]: string | number | boolean}
----@param strs_len integer?
+---@param buff_len integer?
 ---@return integer
-local function extend_strings_with_attrs(strs, attr_map, strs_len)
-	strs_len = strs_len or #strs
+local function extend_str_buff_with_attrs(buff, attr_map, buff_len)
+	buff_len = buff_len or #buff
 	for k, v in pairs(attr_map) do
 		if v == true then
 			-- boolean attributes
 			-- https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes#boolean_attributes
-			strs_len = strs_len + 2
-			strs[strs_len - 1] = ' '
-			strs[strs_len] = k
+			buff_len = buff_len + 2
+			buff[buff_len - 1] = ' '
+			buff[buff_len] = k
 		elseif v then  -- exclude the case `v == false`
-			strs_len = strs_len + 5
-			strs[strs_len - 4] = ' '
-			strs[strs_len - 3] = k
-			strs[strs_len - 2] = '="'
-			strs[strs_len - 1] = utils.attr_encode(tostring(v))
-			strs[strs_len] = '"'
+			buff_len = buff_len + 5
+			buff[buff_len - 4] = ' '
+			buff[buff_len - 3] = k
+			buff[buff_len - 2] = '="'
+			buff[buff_len - 1] = utils.attr_encode(tostring(v))
+			buff[buff_len] = '"'
 		end
 	end
-	return strs_len
+	return buff_len
 end
 
 
@@ -277,7 +281,7 @@ local function elem_to_string(self)
 
 	-- format open tag
 	local result = {'<', tag_name}
-	extend_strings_with_attrs(result, self[SYM_ATTR_MAP])
+	extend_str_buff_with_attrs(result, self[SYM_ATTR_MAP])
 	result[#result+1] = '>'
 
 	-- return without children or close tag when being a void element
@@ -287,7 +291,7 @@ local function elem_to_string(self)
 	end
 
 	-- format children
-	extend_strings_with_fragment(result, self[SYM_CHILDREN], nil, RAW_TEXT_ELEMS[tag_name])
+	extend_str_buff_with_frag(result, self[SYM_CHILDREN], nil, RAW_TEXT_ELEMS[tag_name])
 	-- format close tag
 	result[#result+1] = '</'
 	result[#result+1] = tag_name
@@ -495,7 +499,7 @@ local function breadcrumb_to_string(self)
 		result[#result+1] = '><'
 		result[#result+1] = tag_name
 		if attr_maps[i] then
-			extend_strings_with_attrs(result, attr_maps[i])
+			extend_str_buff_with_attrs(result, attr_maps[i])
 		end
 	end
 	result[1] = '<'
@@ -581,21 +585,21 @@ local function error_wrong_newindex()
 end
 
 
-BareElement_mt = {
+BareElement_mt = node_mts:register {
 	__tostring = bare_elem_to_string,  --> string
 	__index = new_building_elem_by_shorthand_attrs,  --> BuildingElement
 	__call = new_built_elem_from_props,  --> BuiltElement
 	__div = elem_div,  --> Breadcrumb | BuiltElement
 	__newindex = error_wrong_newindex,
 }
-BuildingElement_mt = {
+BuildingElement_mt = node_mts:register {
 	__tostring = elem_to_string,  --> string
 	__call = new_built_elem_from_props,  --> BuiltElement
 	__div = elem_div,  --> Breadcrumb | BuiltElement
 	__index = error_wrong_index,
 	__newindex = error_wrong_newindex,
 }
-BuiltElement_mt = {
+BuiltElement_mt = node_mts:register {
 	__tostring = elem_to_string,  --> string
 	__index = get_elem_prop,
 	__newindex = set_elem_prop,
@@ -603,7 +607,7 @@ BuiltElement_mt = {
 		error('attempt to perform division on a built element', 2)
 	end,
 }
-Breadcrumb_mt = {
+Breadcrumb_mt = node_mts:register {
 	__tostring = breadcrumb_to_string,  --> string
 	__call = function (self, props)  --> BuiltElement
 		local root_elem, leaf_elem = breadcrumb_to_built_elem(self)
@@ -715,7 +719,6 @@ local function to_extended_env(env)
 	return setmt(utils.raw_shallow_copy(env), new_mt)
 end
 
-local classes = require('.classes')
 
 return {
 	-- namespaces
diff --git a/classes.lua b/classes.lua
index aaba113..a8cf931 100644
--- a/classes.lua
+++ b/classes.lua
@@ -4,11 +4,25 @@ local tostring = tostring
 
 local classes = {}
 
+---@type {[metatable]: true, register: fun(self: self, mt: metatable): metatable}
+local node_mts = {
+	---Register a metatable as a node metatable.
+	register = function (self, mt)
+		self[mt] = true
+		return mt
+	end,
+}
+---To judge when a table with `__tostring` is being serialized, whether or not
+---escape the string. If the table is an acandy node, it should not be escaped,
+---as the node itself will handle the escaping.
+classes.node_mts = node_mts
+
 local SYM_CONTENT = {}  ---@type Symbol
 
+
 ---@class Comment
 
-local Comment_mt = {
+local Comment_mt = node_mts:register {
 	__tostring = function (self)
 		return '<!--'..(self[SYM_CONTENT] or '')..'-->'
 	end,
@@ -29,7 +43,7 @@ function classes.Comment(content)
 			or s:find('-->', 1, true)
 			or content:find('--!>', 1, true)
 		then
-			error('invalid comment content: '..content, 2)
+			error('invalid comment content: '..('%q'):format(content), 2)
 		end
 	end
 	return setmt({[SYM_CONTENT] = content}, Comment_mt)
@@ -43,17 +57,22 @@ end
 --   XML: https://www.w3.org/TR/xml/#sec-prolog-dtd
 -- TODO: support any doctype
 
+local Doctype_mt = node_mts:register {
+	__tostring = function ()
+		return '<!DOCTYPE html>'
+	end,
+}
 classes.Doctype = {
-	---HTML5 doctype shortcut. It is a table rather than string for future extension.
+	---HTML5 doctype shortcut.
 	---@type Doctype
-	HTML = setmetatable({}, {
-		__tostring = function () return '<!DOCTYPE html>' end,
-	}),
+	HTML = setmetatable({}, Doctype_mt),
 }
 
 
-local Raw_mt
-Raw_mt = {  ---@type metatable
+---@class Raw
+
+local Raw_mt  ---@type metatable
+Raw_mt = node_mts:register {
 	__tostring = function (self)
 		return self[SYM_CONTENT]
 	end,
@@ -64,13 +83,13 @@ Raw_mt = {  ---@type metatable
 		return setmt({[SYM_CONTENT] = left[SYM_CONTENT]..right[SYM_CONTENT]}, Raw_mt)
 	end,
 	__newindex = function ()
-		error('Raw object is not mutable', 2)
+		error('Raw object is immutable', 2)
 	end,
 }
 
 ---Create a Raw object, which would not be encoded when converted to string.
 ---@param content any value to be converted to string by `tostring()`
----@return table
+---@return Raw
 function classes.Raw(content)
 	return setmt({[SYM_CONTENT] = tostring(content)}, Raw_mt)
 end
diff --git a/tests/acandy_spec.lua b/tests/acandy_spec.lua
index e508091..a01f727 100644
--- a/tests/acandy_spec.lua
+++ b/tests/acandy_spec.lua
@@ -95,35 +95,38 @@ describe('Building element', function ()
 
 	it('can be gotten by indexing a base element with id and/or class', function ()
 		assert.are.equal(tostring(div['my-class']), '<div class="my-class"></div>')
-		assert.are.equal(tostring(div[' my-class ']), '<div class="my-class"></div>')
+		assert.are.equal(tostring(div[' \t\rmy-class\n\f']), '<div class="my-class"></div>')
 
 		assert.are.equal(tostring(div['#my-id']), '<div id="my-id"></div>')
-		assert.are.equal(tostring(div[' #my-id ']), '<div id="my-id"></div>')
+		assert.are.equal(tostring(div[' \t\r#my-id\n\f']), '<div id="my-id"></div>')
 
 		assert.is_true(match_html(
-			tostring(div['#my-id my-class']),
-			'<div', {' id="my-id"', ' class="my-class"'}, '></div>'
+			tostring(div['#my-id my-class-1 my-class-2 my-class-3']),
+			'<div', {' id="my-id"', ' class="my-class-1 my-class-2 my-class-3"'}, '></div>'
 		))
 		assert.is_true(match_html(
-			tostring(div['my-class #my-id']),
-			'<div', {' id="my-id"', ' class="my-class"'}, '></div>'
+			tostring(div['my-class-1 #my-id my-class-2 my-class-3']),
+			'<div', {' id="my-id"', ' class="my-class-1 my-class-2 my-class-3"'}, '></div>'
 		))
 		assert.is_true(match_html(
 			tostring(div['my-class#my-id']),
 			'<div', {' id="my-id"', ' class="my-class"'}, '></div>'
 		))
 		assert.is_true(match_html(
-			tostring(div[' my-class \n #my-id ']),
+			tostring(div[' \t\rmy-class\n\f#my-id ']),
 			'<div', {' id="my-id"', ' class="my-class"'}, '></div>'
 		))
 	end)
 
 	it('raises an error when multiple id provided', function ()
+		assert.has.error(function ()
+			local never = div['##id2']
+		end)
 		assert.has.error(function ()
 			local never = div['#id1#id2']
 		end)
 		assert.has.error(function ()
-			local never = div['#id1 #id2']
+			local never = div['class1 #id1 class2 #id2 class3']
 		end)
 	end)
 
@@ -137,19 +140,19 @@ describe('Building element', function ()
 	it('raises errors when trying to get properties', function ()
 		local elem = div['my-class']
 		assert.has.error(function ()
-			print(elem.tag_name)
+			local never = elem.tag_name
 		end)
 		assert.has.error(function ()
-			print(elem.children)
+			local never = elem.children
 		end)
 		assert.has.error(function ()
-			print(elem.attributes)
+			local never = elem.attributes
 		end)
 		assert.has.error(function ()
-			print(elem[1])
+			local never = elem[1]
 		end)
 		assert.has.error(function ()
-			print(elem.id)
+			local never = elem.class
 		end)
 	end)
 
@@ -219,6 +222,11 @@ describe('Built element', function ()
 		}), '<div class="cls">a1b2c3d4</div>')
 	end)
 
+	it("does not check end tags in raw text elements", function ()
+		assert.are.equal(tostring(a.style('</style>')), '<style></style></style>')
+		assert.are.equal(tostring(a.script('</script>')), '<script></script></script>')
+	end)
+
 	it('has properties', function ()
 		local li = a.li 'item 1'
 		local list = a.ol {id = "my-id", li}
@@ -249,6 +257,11 @@ end)
 
 describe('String escaping', function ()
 	local str = '& \194\160 " \' < > &amp; &nbsp; &quot; &apos; &lt; &gt;'
+	local function func_returns_string()
+		return str
+	end
+	-- issue: #11
+	local table_with_tostring = setmetatable({}, {__tostring = func_returns_string})
 
 	it('replaces `& NBSP " < >` in attribute values with named references', function ()
 		local answer = '<div class="'
@@ -257,7 +270,9 @@ describe('String escaping', function ()
 			..'"></div>'
 		assert.are.equal(tostring(a.div[str]), answer)
 		assert.are.equal(tostring(a.div[{class = str}]), answer)
+		assert.are.equal(tostring(a.div[{class = table_with_tostring}]), answer)
 		assert.are.equal(tostring(a.div {class = str}), answer)
+		assert.are.equal(tostring(a.div {class = table_with_tostring}), answer)
 	end)
 
 	it('replaces `& NBSP < >` in texts other than attribute value with named references', function ()
@@ -266,6 +281,20 @@ describe('String escaping', function ()
 			..'&amp;amp; &amp;nbsp; &amp;quot; &amp;apos; &amp;lt; &amp;gt;'
 			..'</div>'
 		assert.are.equal(tostring(a.div(str)), answer)
+		assert.are.equal(tostring(a.div(func_returns_string)), answer)
+		assert.are.equal(tostring(a.div(table_with_tostring)), answer)
+	end)
+
+	it('does not escape string from acandy nodes', function ()
+		local node1 = a.div
+		assert.are.equal(tostring(a.div(node1)), '<div>'..tostring(node1)..'</div>')
+		local node2 = acandy.Doctype.HTML
+		assert.are.equal(
+			tostring(Fragment {node2, a.html {a.head, a.body}}),
+			tostring(node2)..'<html><head></head><body></body></html>'
+		)
+		local node3 = acandy.Comment(str)
+		assert.are.equal(tostring(a.div(node3)), '<div>'..tostring(node3)..'</div>')
 	end)
 
 	it("does not replace characters in an object's properties", function ()
@@ -276,18 +305,94 @@ describe('String escaping', function ()
 		assert.are.equal(elem.class, str)
 		assert.are.equal(elem[1], str)
 	end)
+
+	it("does not encode any character in raw text elements", function ()
+		assert.are.equal(tostring(a.style(str)), '<style>'..str..'</style>')
+		assert.are.equal(tostring(a.script(str)), '<script>'..str..'</script>')
+	end)
 end)
 
-describe('Raw text element', function ()
+describe('`acandy.Comment`', function ()
+	-- spec: https://html.spec.whatwg.org/#comments
+
+	local Comment = acandy.Comment
+	it('arg is content', function ()
+		local comment = Comment('<>&"')
+		assert.are.equal(tostring(comment), '<!--<>&"-->')
+		assert.are.equal(tostring(a.div(comment)), '<div><!--<>&"--></div>')
+	end)
+
+	it('raises an error when content is invalid', function ()
+		-- the text must not start with the string ">" or "->",
+		-- nor contain the strings "<!--", "-->", or "--!>",
+		-- nor end with the string "<!-".
+		for _, invalid_content in ipairs {
+			'>', '>text', '->', '->text',
+			'<!--', 'text<!--', '<!--text', 'text<!--text',
+			'-->', 'text-->', '-->text', 'text-->text',
+			'--!>', 'text--!>', '--!>text', 'text--!>text',
+			'<!-', 'text<!-',
+		} do
+			assert.has.error(function ()
+				local never = Comment(invalid_content)
+			end)
+		end
+
+		for _, valid_content in ipairs {
+			'text>', 'text>text', 'text->', 'text->text',
+			'<!-text', 'text<!-text',
+			-- the text is allowed to end with the string "<!",
+			-- as in <!--My favorite operators are > and <!-->.
+			'<!', 'text<!', '<!text', 'text<!text',
+		} do
+			assert.is.equal(tostring(Comment(valid_content)), '<!--'..valid_content..'-->')
+		end
+	end)
+end)
+
+describe('`acandy.Doctype`', function ()
+	-- spec: https://html.spec.whatwg.org/#the-doctype
+
+	local Doctype = acandy.Doctype
+	it('has HTML5 doctype', function ()
+		assert.are.equal(tostring(Doctype.HTML), '<!DOCTYPE html>')
+		assert.are.equal(tostring(Fragment {Doctype.HTML, a.html}), '<!DOCTYPE html><html></html>')
+	end)
+end)
+
+describe('`acandy.Raw`', function ()
 	local str = '& \194\160 " \' < > &amp; &nbsp; &quot; &apos; &lt; &gt;'
+	it('is not escaped as a child node', function ()
+		local raw = acandy.Raw(str)
+		assert.are.equal(tostring(raw), str)
+		assert.are.equal(tostring(a.div(raw)), '<div>'..str..'</div>')
+		assert.are.equal(tostring(a.div {raw}), '<div>'..str..'</div>')
+	end)
 
-	it("does not encode any character in text child", function ()
-		assert.are.equal(tostring(a.style(str)), '<style>'..str..'</style>')
-		assert.are.equal(tostring(a.script(str)), '<script>'..str..'</script>')
+	it('can concat with another `Raw`, returns `Raw`', function ()
+		local str1, str2 = '1'..str, '2'..str
+		local raw1 = acandy.Raw(str1)
+		local raw2 = acandy.Raw(str2)
+		assert.are.equal(getmetatable(raw1), getmetatable(raw2), getmetatable(raw1..raw2))
+		assert.are.equal(tostring(raw1..raw2), str1..str2)
+		assert.are.equal(tostring(a.div(raw1..raw2)), '<div>'..str1..str2..'</div>')
+		assert.are.equal(tostring(raw1..raw2..raw1), str1..str2..str1)
+		assert.are.equal(tostring(a.div(raw1..raw2..raw1)), '<div>'..str1..str2..str1..'</div>')
 	end)
 
-	it("does not check end tag", function ()
-		assert.are.equal(tostring(a.style('</style>')), '<style></style></style>')
-		assert.are.equal(tostring(a.script('</script>')), '<script></script></script>')
+	it('cannot concat with a string or number', function ()
+		local raw = acandy.Raw(str)
+		assert.has.error(function ()
+			local never = raw..str
+		end)
+		assert.has.error(function ()
+			local never = str..raw
+		end)
+		assert.has.error(function ()
+			local never = raw..1
+		end)
+		assert.has.error(function ()
+			local never = (1)..raw
+		end)
 	end)
 end)
diff --git a/utils.lua b/utils.lua
index 0c83015..78e5d0e 100644
--- a/utils.lua
+++ b/utils.lua
@@ -203,10 +203,10 @@ function utils.parse_shorthand_attrs(str)
 	local id = nil
 	str = s_gsub(str, '#([^%s#]*)', function (s)
 		if s == '' then
-			error('empty id found in '..string.format('%q', str), 4)
+			error('empty id found in '..('%q'):format(str), 4)
 		end
 		if id then
-			error('two or more ids found in '..string.format('%q', str), 4)
+			error('two or more ids found in '..('%q'):format(str), 4)
 		end
 		id = s
 		return ''