Role of <merror>
Marks content as an error representation. Use the element for its semantic role rather than as a visual styling shortcut. Assistive technology, browser math layout, conversion tools, and copy-and-paste behavior all depend on the markup preserving the mathematical structure.
Example markup
<merror><mtext>Invalid expression</mtext></merror>Allowed children
presentation MathML
Only place children inside <merror> when they match the element's content model. Validate the complete MathML tree after conversion because a locally valid fragment can still be incorrect in its parent context.
Implementation notes
- Provide a useful message rather than exposing parser internals.
Accessibility and testing
- Keep a meaningful DOM structure instead of replacing the expression with an image.
- Test keyboard selection, screen-reader output, browser rendering, and conversion back to LaTeX.
- Check that fallbacks do not expose raw tags or omit the mathematical relationship represented by <merror>.