Marks content as a warning box. Renders with a "Warning" heading and distinct styling to draw attention to important cautions.

<div class="warning">
  <p>Removing entries from this list is a breaking change.</p>
</div>

<aside class="warning" title="Deprecation notice">
  <p>The <code>sync</code> flag has been deprecated and will be removed in a future version.</p>
</aside>

• Works on block elements: <div>, <p>, <aside>, <section>
• The title attribute is interpreted as HTML — see title attributes
• Warnings appear in the issue-summary section alongside issues and notes
• For issues that editors are tracking, use .issue instead
• For non-normative editorial notes, use .ednote instead