The void-tag pass from the previous commit fixes `<br>`, but mermaid's
label HTML carries more than void tags: a non-breaking space (` `,
mermaid's `#nbsp;`, or a literal U+00A0 in the source) is serialised as
the ` ` entity, which XML does not define. Such a label still
decodes to a broken image.
mermaid.render() returns the HTML serialisation of the diagram, so parse
it with the HTML parser it was written for and re-serialise it with
XMLSerializer. That fixes the whole class at once. Against mermaid
11.16 in Chromium, across flowchart, class, state, ER, mindmap, block,
kanban, sequence, journey, gantt, pie, timeline, gitGraph, quadrant and
xychart output, the round-trip yields the same element tree and image
size as the regex wherever the regex output is valid XML, and also loads
the non-breaking-space cases the regex leaves broken.
Drop a label's plain `xmlns` attribute before serialising: the
serializer declares the namespace itself, and a spec-conformant one
(jsdom) writes both, a duplicate attribute.
The tests' fixtures put `<br>` directly in SVG <text>, which mermaid
never emits; replace them with the real 11.16 label shape (XHTML in a
<foreignObject>) and assert the label keeps its line break and its
non-breaking space.
Refs #133089, #134031
2af85c331a