Simon Willison’s Weblog

14th March 2025 - Link Blog

TIL: Styling an HTML dialog modal to take the full height of the viewport. I spent some time today trying to figure out how to have a modal <dialog> element present as a full height side panel that animates in from the side. The full height bit was hard, until Natalie helped me figure out that browsers apply a default max-height: calc(100% - 6px - 2em); rule which needs to be over-ridden.

Also included: some spelunking through the HTML spec to figure out where that calc() expression was first introduced. The answer was November 2020.