Add runScripts to SetHTMLUnsafeOptions

[noamr]

When runScripts is true, scripts are executed after the DOM is updated. This uses the "scripting mode" mechanism used by createContextualFragment.

Closes #10090

• At least two implementers are interested (and none opposed):
  • Chromium
  • …
• Tests are written and can be reviewed and commented upon at:
  • Remove .tentative from setHTMLUnsafe-runScripts test web-platform-tests/wpt#60234
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/469706054
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2043150
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=315741
• MDN issue is filed: this is done as part of chromium shipping process.
• The top of this comment includes a clear commit message to use.

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

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/dom.html ( diff )
/dynamic-markup-insertion.html ( diff )

[noamr]

Re multiple implementer support - I've filed WebKit/standards-positions#629 and
mozilla/standards-positions#1370 for the whole picture of HTML updates, which should cover this as well. cc @annevk @zcorpan .