In Chromium it's quite literally a CSS animation: https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/html/html_marquee_ele

In Chromium it's quite literally a CSS animation: https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/html/html_marquee_element.cc;l=241;drc=eac3931ab001973d939a36f424027b3e5f81102a 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).