[whatwg/url] Add Unicode ToASCII fallback for ASCII domains (PR #914)

Unfortunately implementing Unicode ToASCII ends up rejecting websites such as

- http://xn--72czcrhaj7cpt0ed1dxb4mb1s1.blogspot.com/2018/11/blog-post_60.html
- https://xn--8i7caa.famitei.net/sekou/all
- https://xn--board-ngr.palungjit.org/members/

which is not really acceptable. So when ToASCII fails, domain is all ASCII, and there are no forbidden domain code points, we just return the lowercased domain. If an implementation does not surface validation errors this is indistinguishable from having an ASCII fast path.

Tests: https://github.com/web-platform-tests/wpt/pull/60476

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<a href="https://whatpr.org/url/914.html" title="Last updated on Jun 8, 2026, 7:02 PM UTC (270cd1f)">Preview</a> | <a href="https://whatpr.org/url/914/9fe73be...270cd1f.html" title="Last updated on Jun 8, 2026, 7:02 PM UTC (270cd1f)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/url/pull/914

-- Commit Summary --

  * Add Unicode ToASCII fallback for ASCII domains

-- File Changes --

    M url.bs (36)

-- Patch Links --

https://github.com/whatwg/url/pull/914.patch
https://github.com/whatwg/url/pull/914.diff

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