Intent to prototype: Escape "<" and ">" in attributes when serializing HTML

Summary: We want to change the escaping of HTML attribute values to
include "<" (&lt;) and ">" (&gt).
The fact that these characters are not escaped currently can lead to
security issues in HTML parsers and sanitizers. (Currently
Nightly-only)
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1941347
Specification: https://github.com/whatwg/html/pull/6362
Standards Body: WhatWG
Platform coverage: everywhere
Preference: dom.security.html_serialization_escape_lt_gt
DevTools bug: n/a
Link to standards-positions discussion: none
Other browsers:
- Chrome: Rollout on Stable
https://github.com/whatwg/html/issues/6235#issuecomment-2729072764
- Webkit: no information

https://github.com/mozilla/standards-positions/issues/1209