Simon Willison’s Weblog

First look at the modern attr(). Chrome 133 (released February 25th 2025) was the first browser to ship support for the advanced CSS attr() function (MDN), which lets attr() be used to compose values using types other than strings.

Ahmad Shadeed explores potential applications of this in detail, trying it out for CSS grid columns, progress bars, background images, animation delays and more.

I like this example that uses the rows="5" attribute on a <textarea> to calculate its max-height - here wrapped in a feature detection block:

@supports (x: attr(x type(*))) {
  textarea {
    min-height: calc(
      attr(rows type(<number>)) * 50px
    );
  }
}

That type(<number>) is the new syntax.

Many of Ahmad's examples can be achieved today across all browsers using a slightly more verbose CSS custom property syntax.

Here are the tracking issues for CSS values support in attr() for Firefox (opened 17 years ago) and WebKit (16 years ago).