HTML API: Ensure correct serialization of XMP contents

[sirreal]

XMP contents are not decoded and should not use HTML character reference escaping when serializing.

Escaping on serialize does not preserve the contents, so sequential normalizations of the original become:

1. <xmp> < > & " ' ␀ </xmp> (original, ␀ is U+0000 NUL)
2. <xmp> &lt; &gt; &amp; &quot; &apos; � </xmp>
3. <xmp> &amp;lt; &amp;gt; &amp;amp; &amp;quot; &amp;apos; � </xmp>
4. etc…

(Note that the NUL -> � replacement is correct).

See this demo

Note that XMP is an obsolete element, however it is still present in the HTML5 specification.

Trac ticket: https://core.trac.wordpress.org/ticket/65372

Testing

Compare the HTML API debugger example above on 7.0 with this branch.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude (Opus 4.8, Fable 5), Codex (GPT-5.5)
Used for: Fuzz test implementation, analysis of defects, fix implementation, review.

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