Revise hardwareAcceleration based on consensus decision.

* It's now a hint instead of being required.
* Values are noPreference, preferSoftware, and preferHardware.

Fixes: #239

index.src.html

@@ -1630,7 +1630,7 @@
   [EnforceRange] unsigned long codedHeight;
   [EnforceRange] unsigned long displayAspectWidth;
   [EnforceRange] unsigned long displayAspectHeight;
-  HardwareAcceleration hardwareAcceleration = "allow";
+  HardwareAcceleration hardwareAcceleration = "noPreference";
   boolean optimizeForLatency;
 };
 </xmp>
@@ -1697,7 +1697,7 @@
 
   <dt><dfn dict-member for=VideoDecoderConfig>hardwareAcceleration</dfn></dt>
   <dd>
-    Configures hardware acceleration for this codec. See
+    Hint that configures hardware acceleration for this codec. See
     {{HardwareAcceleration}}.
   </dd>
 
@@ -1762,7 +1762,7 @@
   [EnforceRange] unsigned long displayHeight;
   [EnforceRange] unsigned long long bitrate;
   [EnforceRange] double framerate;
-  HardwareAcceleration hardwareAcceleration = "allow";
+  HardwareAcceleration hardwareAcceleration = "noPreference";
   AlphaOption alpha = "discard";
   DOMString scalabilityMode;
   BitrateMode bitrateMode = "variable";
@@ -1849,7 +1849,7 @@
 
   <dt><dfn dict-member for=VideoEncoderConfig>hardwareAcceleration</dfn></dt>
   <dd>
-    Configures hardware acceleration for this codec. See
+    Hint that configures hardware acceleration for this codec. See
     {{HardwareAcceleration}}.
   </dd>
 
@@ -1885,30 +1885,41 @@
 ---------------------------------------------
 <xmp class='idl'>
 enum HardwareAcceleration {
-  "allow",
-  "deny",
-  "require",
+  "noPreference",
+  "preferHardware",
+  "preferSoftware",
 };
 </xmp>
 
 When supported, hardware acceleration offloads encoding or decoding to
-specialized hardware.
+specialized hardware. {{HardwareAcceleration/preferHardware}} and
+{{HardwareAcceleration/preferSoftware}} are hints. While User Agents SHOULD
+respect these values when possible, User Agents may ignore these values in some
+or all circumstances for any reason.
 
 <div class='note'>
+  NOTE: Good examples of when a User Agent may ignore
+  {{HardwareAcceleration/preferHardware}} or
+  {{HardwareAcceleration/preferSoftware}} are for reasons of user privacy or
+  circumstances where the User Agent determines an alternative setting would
+  better serve the end user.
+
   NOTE: Most authors will be best served by using the default of
-  {{HardwareAcceleration/allow}}. This gives the User Agent flexibility to
-  optimize based on its knowledge of the system and configuration. A common
+  {{HardwareAcceleration/noPreference}}. This gives the User Agent flexibility
+  to optimize based on its knowledge of the system and configuration. A common
   strategy will be to prioritize hardware acceleration at higher resolutions
   with a fallback to software codecs if hardware acceleration fails.
 
-  Authors should carefully weigh the tradeoffs when setting a hardware acceleration
-  preference. The precise tradeoffs will be device-specific, but authors should
-  generally expect the following:
+  Authors should carefully weigh the tradeoffs when setting a hardware
+  acceleration preference. The precise tradeoffs will be device-specific, but
+  authors should generally expect the following:
 
-  * Setting a value of {{HardwareAcceleration/require}} may significantly
-      restrict what configurations are supported. It may occur that the user's
-      device does not offer acceleration for any codec, or only for the most
-      common profiles of older codecs.
+  * Setting a value of {{HardwareAcceleration/preferHardware}} or
+      {{HardwareAcceleration/preferSoftware}} may significantly restrict what
+      configurations are supported. It may occur that the user's device does not
+      offer acceleration for any codec, or only for the most common profiles of
+      older codecs. It may also occur that a given User Agent lacks a software
+      based codec implementation.
   * Hardware acceleration does not simply imply faster encoding / decoding.
       Hardware acceleration often has higher startup latency but more consistent
       throughput performance. Acceleration will generally reduce CPU load.
@@ -1920,33 +1931,35 @@
       may yield decreased performance and power efficiency compared to purely
       software based codecs.
 
-  Given these tradeoffs, a good example of using "require" would be if an author
-  intends to provide their own software based fallback via WebAssembly.
+  Given these tradeoffs, a good example of using "preferHardware" would be if an
+  author intends to provide their own software based fallback via WebAssembly.
 
-  Alternatively, a good example of using "disallow" would be if an author is
-  especially sensitive to the higher startup latency or decreased robustness
+  Alternatively, a good example of using "preferSoftware" would be if an author
+  is especially sensitive to the higher startup latency or decreased robustness
   generally associated with hardware acceleration.
 </div>
 
 <dl>
-  <dt><dfn enum-value for=HardwareAcceleration>allow</dfn></dt>
+  <dt><dfn enum-value for=HardwareAcceleration>noPreference</dfn></dt>
   <dd>
     Indicates that the User Agent may use hardware acceleration if it is
     available and compatible with other aspects of the codec configuration.
   </dd>
-  <dt><dfn enum-value for=HardwareAcceleration>deny</dfn></dt>
+  <dt><dfn enum-value for=HardwareAcceleration>preferSoftware</dfn></dt>
   <dd>
-    Indicates that the User Agent must not use hardware acceleration.
+    Indicates that the User Agent SHOULD prefer a software codec implementation.
+    User Agents may ignore this value for any reason.
 
-    NOTE: This will cause the configuration to be unsupported on platforms where
+    NOTE: This may cause the configuration to be unsupported on platforms where
     an unaccelerated codec is unavailable or is incompatible with other aspects
     of the codec configuration.
   </dd>
-  <dt><dfn enum-value for=HardwareAcceleration>require</dfn></dt>
+  <dt><dfn enum-value for=HardwareAcceleration>preferHardware</dfn></dt>
   <dd>
-    Indicates that the User Agent must use hardware acceleration.
+    Indicates that the User Agent SHOULD prefer hardware acceleration. User
+    Agents may ignore this value for any reason.
 
-    NOTE: This will cause the configuration to be unsupported on platforms where
+    NOTE: This may cause the configuration to be unsupported on platforms where
     an accelerated codec is unavailable or is incompatible with other aspects of
     the codec configuration.
   </dd>