In Chromium it's quite literally a CSS animation: https://source.chromium.org/ch...

esprehn 38 days ago | parent | context | favorite | on: <Blink> and <Marquee> (2020)

In Chromium it's quite literally a CSS animation: https://source.chromium.org/chromium/chromium/src/+/main:thi...

All the code in that file is a fun read. <marquee> was rewritten as if it was a web component using the public DOM APIs (ex. It uses CSS animations and requestAnimationFrame).