handle xhtml void elements in mermaid diagrams (#2103)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -1483,6 +1483,7 @@ raw template
 {%- endblock in_prompt -%}
     """
 
+
 exporter_attr = AttrExporter()
 output_attr, _ = exporter_attr.from_notebook_node(nb)
 assert "raw template" in output_attr

share/templates/lab/mermaidjs.html.j2

@@ -84,7 +84,8 @@ url="https://cdnjs.cloudflare.com/ajax/libs/mermaid/10.7.0/mermaid.esm.min.mjs"
       let results = null;
       let output = null;
       try {
-        const { svg } = await mermaid.render(id, raw, el);
+        let { svg } = await mermaid.render(id, raw, el);
+        svg = cleanMermaidSvg(svg);
         results = makeMermaidImage(svg);
         output = document.createElement("figure");
         results.map(output.appendChild, output);
@@ -99,6 +100,38 @@ url="https://cdnjs.cloudflare.com/ajax/libs/mermaid/10.7.0/mermaid.esm.min.mjs"
       parent.appendChild(output);
     }
 
+
+    /**
+     * Post-process to ensure mermaid diagrams contain only valid SVG and XHTML.
+     */
+    function cleanMermaidSvg(svg) {
+      return svg.replace(RE_VOID_ELEMENT, replaceVoidElement);
+    }
+
+
+    /**
+     * A regular expression for all void elements, which may include attributes and
+     * a slash.
+     *
+     * @see https://developer.mozilla.org/en-US/docs/Glossary/Void_element
+     *
+     * Of these, only `<br>` is generated by Mermaid in place of `\n`,
+     * but _any_ "malformed" tag will break the SVG rendering entirely.
+     */
+    const RE_VOID_ELEMENT =
+      /<\s*(area|base|br|col|embed|hr|img|input|link|meta|param|source|track|wbr)\s*([^>]*?)\s*>/gi;
+
+    /**
+     * Ensure a void element is closed with a slash, preserving any attributes.
+     */
+    function replaceVoidElement(match, tag, rest) {
+      rest = rest.trim();
+      if (!rest.endsWith('/')) {
+        rest = `${rest} /`;
+      }
+      return `<${tag} ${rest}>`;
+    }
+
     void Promise.all([...diagrams].map(renderOneMarmaid));
   });
 </script>