Update fetchPriority to match the final spec

[pmeenan]

Description

This updates the fetchpriority HTML attribute and priority fetch option to match the final spec. This is a cleaner version of this PR that had rebase issues.

Motivation

The Priority Hints spec was merged into the HTML and fetch specs, obsoleting the original spec.

Additional details

Here is the HTML spec PR and the Fetch spec PR.

Summarized changes:

• The iFrame element no longer supports the fetchPriority attribute (was never implemented anywhere).
• Remove the priority attribute from the Request interface (it is only settable through options).
• The priority option as part of creating a fetch request and constructing a request was updated.
• A stray reference to the old importance attribute was updated.

Related issues and pull requests

Fixes #24519

[github-actions]

Preview URLs (6 pages)

• /en-US/docs/Learn/Performance/Multimedia
• /en-US/docs/Web/API/Request/Request
• /en-US/docs/Web/API/Request
• /en-US/docs/Web/API/fetch
• /en-US/docs/Web/HTML/Attributes
• /en-US/docs/Web/HTML/Element/iframe

Flaws (2)

Note! 4 documents with no flaws that don't need to be listed. 🎉

URL: /en-US/docs/Web/API/Request/Request
Title: Request()
Flaw count: 1

• broken_links:
  • Can't resolve /en-US/docs/Web/API/abortsignal

---

URL: /en-US/docs/Web/HTML/Element/iframe
Title: <iframe>: The Inline Frame element
Flaw count: 1

• broken_links:
  • Can't resolve /en-US/docs/Learn/Common_questions/What_is_a_URL

External URLs (7)

URL: /en-US/docs/Web/HTML/Attributes
Title: HTML attribute reference

• https://dom.spec.whatwg.org/ (1 time)
• https://html.spec.whatwg.org/dev/interaction.html (1 time)
• https://html.spec.whatwg.org/multipage/common-microsyntaxes.html (1 time)
• https://html.spec.whatwg.org/multipage/urls-and-fetching.html (2 times)
• https://w3c.github.io/html-media-capture/ (1 time)

---

URL: /en-US/docs/Web/HTML/Element/iframe
Title: <iframe>: The Inline Frame element

• https://example.org (1 time)
• https://www.w3.org/TR/html-aria/ (1 time)