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.

# 11th April 2025, 3:54 am / browsers, chrome, css, firefox, html, mozilla, safari, web-standards

Backstory on the default styles for the HTML dialog modal. My TIL about Styling an HTML dialog modal to take the full height of the viewport (here's the interactive demo) showed up on Hacker News this morning, and attracted this fascinating comment from Chromium engineer Ian Kilpatrick.

There's quite a bit of history here, but the abbreviated version is that the dialog element was originally added as a replacement for window.alert(), and there were a libraries polyfilling dialog and being surprisingly widely used.

The mechanism which dialog was originally positioned was relatively complex, and slightly hacky (magic values for the insets).

Changing the behaviour basically meant that we had to add "overflow:auto", and some form of "max-height"/"max-width" to ensure that the content within the dialog was actually reachable.

The better solution to this was to add "max-height:stretch", "max-width:stretch". You can see the discussion for this here.

The problem is that no browser had (and still has) shipped the "stretch" keyword. (Blink likely will "soon")

However this was pushed back against as this had to go in a specification - and nobody implemented it ("-webit-fill-available" would have been an acceptable substitute in Blink but other browsers didn't have this working the same yet).

Hence the calc() variant. (Primarily because of "box-sizing:content-box" being the default, and pre-existing border/padding styles on dialog that we didn't want to touch). [...]

I particularly enjoyed this insight into the challenges of evolving the standards that underlie the web, even for something this small:

One thing to keep in mind is that any changes that changes web behaviour is under some time pressure. If you leave something too long, sites will start relying on the previous behaviour - so it would have been arguably worse not to have done anything.

Also from the comments I learned that Firefox DevTools can show you user-agent styles, but that option is turned off by default - notes on that here. Once I turned this option on I saw references to an html.css stylesheet, so I dug around and found that in the Firefox source code. Here's the commit history for that file on the official GitHub mirror, which provides a detailed history of how Firefox default HTML styles have evolved with the standards over time.

And via uallo here are the same default HTML styles for other browsers:

• Chromium: third_party/blink/renderer/core/html/resources/html.css
• WebKit: Source/WebCore/css/html.css

# 16th March 2025, 4:36 pm / chrome, css, firefox, html, web-standards

Plugging the CSS History Leak (via) Firefox is fixing the nefarious CSS visited link colour history leak flaw, which currently affects all browsers and allows a malicious site to determine if you have visited a specific site by checking getComputedStyle against a link to that page. It’s an obtrusive but necessary fix—visited link styles will be restricted to colour and border styles (no background images and hence no more checkbox effects since the image request could leak information) and those colours will not be reported via getComputedStyle. I hope other browser vendors follow suit.

# 31st March 2010, 8:01 pm / css, firefox, history, mozilla, security

FireScope. Neat little Firefox / Firebug extension which adds a “Reference” tab showing documentation for the selected element from the comprehensive SitePoint Reference site.

# 5th February 2009, 10:51 pm / css, documentation, extensions, firebug, firefox, firescope, html, reference, sitepoint

When Bugs Collide: Fixing Text Dimming in Firefox 2. Handy tips from Drew on fixing the glitchy text rendering in Firefox 2 when you animate opacity without breaking alpha-transparent PNGs in IE6.

# 19th June 2008, 6:09 pm / browsers, bugs, css, drew-mclellan, firefox, firefox2, ie6, javascript, jquery, opacity, pngs