Copy a document's base URL when cloning

[annevk]

Copy a document's base URL when cloning

The baseURI getter now defers to a new "get the base URL" algorithm, which returns null by default and falls back to the node document's URL. HTML overrides this algorithm (to return the document base URL), so DOM no longer reaches into HTML for a value returned by a DOM API.

Cloning a document now copies its base URL into a new "base URL override", so that a shallow clone, which has no base element, still reports the base URL of the document it was cloned from.

Fixes #454.

Tests: web-platform-tests/wpt#61232

• At least two implementers are interested (and none opposed):
  • Gecko (passes all tests)
  • WebKit
• Tests are written and can be reviewed and commented upon at:
  • Cloning a document copies its base URL web-platform-tests/wpt#61232
• Implementation bugs are filed:
  • Chromium: Captcha :-(
  • Gecko: N/A
  • WebKit: …
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

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

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