Simon Willison’s Weblog

Default styles for h1 elements are changing (via) Wow, this is a rare occurrence! Firefox are rolling out a change to the default user-agent stylesheet for nested <h1> elements, currently ramping from 5% to 50% of users and with full roll-out planned for Firefox 140 in June 2025. Chrome is showing deprecation warnings and Safari are expected to follow suit in the future.

What's changing? The default sizes of <h1> elements that are nested inside <article>, <aside>, <nav> and <section>.

These are the default styles being removed:

/* where x is :is(article, aside, nav, section) */
x h1 { margin-block: 0.83em; font-size: 1.50em; }
x x h1 { margin-block: 1.00em; font-size: 1.17em; }
x x x h1 { margin-block: 1.33em; font-size: 1.00em; }
x x x x h1 { margin-block: 1.67em; font-size: 0.83em; }
x x x x x h1 { margin-block: 2.33em; font-size: 0.67em; }

The short version is that, many years ago, the HTML spec introduced the idea that an <h1> within a nested section should have the same meaning (and hence visual styling) as an <h2>. This never really took off and wasn't reflected by the accessibility tree, and was removed from the HTML spec in 2022. The browsers are now trying to cleanup the legacy default styles.

This advice from that post sounds sensible to me:

• Do not rely on default browser styles for conveying a heading hierarchy. Explicitly define your document hierarchy using <h2> for second-level headings, <h3> for third-level, etc.
• Always define your own font-size and margin for <h1> elements.