Make muted content attribute affect muted IDL attribute only once

[foolip]

The purpose of this is to limit the cases where the muted IDL attribute
can change without a volumechange event firing.

The notion that user agents may remember volume and muted is removed as
it would require additional muted states to handle correctly, but could
be reinstated if there is implementer interest.

• At least two implementers are interested (and none opposed):
  • Chromium (@foolip)
  • …
• Tests are written and can be reviewed and commented upon at:
  • https://crrev.com/c/7857861
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

---

/media.html ( diff )

[annevk]

What do implementations do today? I'm pretty sure WebKit doesn't have a "set once" special case.

[foolip]

Chromium reacts to muted content attribute changes but only for AttributeModificationReason::kByParser which can only happen once in HTMLMediaElement::ParseAttribute().

Gecko seems to look at the muted content attribute in two places: in HTMLMediaElement::DoneCreatingElement() and in HTMLMediaElement::CopyInnerTo(), which maybe is for cloning, not sure.

WebKit has a tri-state optional bool m_implicitlyMuted and a boolean m_explicitlyMuted. I'm not sure about m_implicitlyMuted, but m_explicitlyMuted is used as a once-only state bit when setting m_muted from the attribute in two places: in MediaElement::postConnectionSteps() and in MediaElement::loadResource().

[foolip]

For Gecko, the MutedReasons is also interesting and confirms that playbackRate can mute in Gecko. Seems like that doesn't fire a "volumechange" event.

[foolip]

cc @whatwg/media for opinions on this. In short, the topic is how/when the muted content attribute affects the muted IDL attribute, and which changes in the muted IDL attribute should fire a volumechange event.

Recent changes here are #12389 and #12428.

[annevk]

I see, perhaps observing the muted content attribute in the post-connection steps is a good alternative to the fictional "creation time". As that is a well-defined point in time.

That would require changing the :muted tests again I suspect.

[foolip]

@annevk that works for me, I've updated this PR and tests in https://crrev.com/c/7857861 to match.

[annevk]

What about the WebKit case of taking the muted content attribute into account when loading a new media resource?