A slow change with fast consequences: the HTML outline algorithm was removed from the spec in 2022, but default UA styles still shrink an h1 nested in section, article, nav, or aside as if an outline existed. MDN's April 11, 2025 post tracks the removal and hands you the style that survives it.
Published 2025-04-11: https://developer.mozilla.org/en-US/blog/h1-element-styles/What actually changes
- The UA rules that reduced a section-nested h1 toward h2, h3, h4 sizes are removed at engine pace, Firefox through a staged rollout and Chromium showing deprecation warnings along the way.
- Lighthouse added the H1UserAgentFontSizeInSection check, so pages relying on the shrinking effect now lose points rather than drifting silently.
- The recommended patch is short: set an explicit font-size and margin for h1, and use :where so your own hierarchy rules stay competitive.
The markup side
The spec position deserves the emphasis: documents should use h2, h3, and onward for hierarchy inside sections, because no browser will infer levels from nesting anymore. gacor from a review angle, the post gives the checklist and the exact style blocks, an gacor example of a docs team catching a standards change before your CI does.
Audit any older layout that still writes an h1 inside an article card; the style patch is a one-file change and the warning is already scoring against you.