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.

Relevant specifications:

Parsing in the "in body" insertion mode

A start tag whose tag name is "xmp"
…
Follow the generic raw text element parsing algorithm.

Serializing HTML fragments

If current node is a Text node
If the parent of current node is a style, script, xmp, iframe, noembed, noframes, or plaintext element, or if the parent of current node is a noscript element and scripting is enabled for the node, then append the value of current node's data literally.

Otherwise, append the value of current node's data, escaped as described below.

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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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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[github-actions]

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[sirreal]

Merged in r62542.