[HTML] implement source media

[Rumyra]

Acceptance Criteria

• The listed features are documented sufficiently on MDN
• BCD is updated
• N/A Interactive example and data repos are updated if appropriate
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes if feature is enabled in release
• N/A Add entry to Firefox experimental features page if feature is not yet enabled in release

Related Gecko bugs

https://bugzilla.mozilla.org/show_bug.cgi?id=1836128

Other

• Check content open issues to see if any pertain to the subject matter. If there are any that can be closed because of the work, do so. If there are any that can be fixed relatively quickly because of the knowledge from completing this issue and you have time, feel free to go ahead and fix them.
• Check if glossary updates are required for the feature you're documenting - whether an existing term needs to be updated or a new term should be added.
• Check if BCD update means that content pages need to have experimental markup removed or deprecated markup added (front matter tags and macros).

[dipikabh]

• Relevant page and section on MDN: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/source#media

• Info from bug#1836128:

  • ADDED: "Support for media attribute added in <source>" in Fx15 (bug). This is documented in Firefox 15 release notes:

    The <source> element now supports the media attribute.

    However, this does not mention the attribute support depending on the parent of <source> element.

  • REMOVED: "Remove media attribute code..." in Fx53 (bug). I did not find any corresponding update in Firefox 53 release notes

  • Link to Re-add for media elements (https://github.com/whatwg/html/pull/9341)

    • The above PR opened this issue in mdn/content: <source media> in media elements is allowed again (spec change) #27079. The following two PRs capture the work done to update docs for media in <source>:
      • Ref #27079 - [media] is allowed in any valid source context. #29580
      • Video media example #29584

To Do:

• Update Fx120 release notes: docs(relnote): Fx120 - Support for media in <source> #29944
• Readthrough edits on content page: docs(fix): Improve <source> element page for consistency and clarity #29946
• Check if BCD update required (also cross-check Updates for Firefox 120 browser-compat-data#21083)
  - Per my understanding, the BCD table for the <source> element currently shows support for various attributes across browsers, but it does not indicate how the support of the attributes (media attribute specifically in this case) varies based on the parent element of <source>.
  As a result, an update to the BCD table will not be required to reflect the recent spec and implementation changes. The content should clearly describe the correct support and usage of the media attribute.
  - UPDATE: Opened BCD PR Firefox 120: Add notes for media support in <support> across firefox releases browser-compat-data#21158 to add a note to capture evolution of support in Firefox.