Track XHTML metadata elements inside SVG

[kjanat]

Context

SVG documents can include HTML metadata elements by switching namespace, e.g.:

<svg xmlns="http://www.w3.org/2000/svg">
  <link xmlns="http://www.w3.org/1999/xhtml" rel="stylesheet" href="style.css" />
</svg>

That element is XHTML {http://www.w3.org/1999/xhtml}link, not SVG <link>.

Refs:

• SVG 2 HTML metadata elements: https://www.w3.org/TR/SVG2/struct.html#HTMLMetadataElements
• HTML <link> docs: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/link

Follow-up

• Verify parser/linter namespace handling for foreign metadata elements inside SVG.
• Decide whether catalog/docs should expose XHTML metadata elements distinctly from SVG elements.
• Add regression coverage for namespaced <link> so it is not treated as unknown SVG <link>.

Notes

Plain <link> without the XHTML namespace remains in SVG namespace and should not be accepted as SVG metadata.