A few things I’ve been using without EdgeHTML support: #rgba and #rrggbbaa in CSS, <details>, Custom Elements v1. On my personal website, I have deliberately chosen to no longer polyfill these or change my process to avoid #rgba in the final CSS, so various background colours are missing, the light/dark theme switcher doesn’t collapse (it should start collapsed), and a terminal recording will be missing in an upcoming blog post.

mix-blend-mode is another thing that EdgeHTML doesn’t support that I was using for a bit, but I stopped using it because I kept finding critical bugs with it in Chromium, most notably https://bugs.chromium.org/p/chromium/issues/detail?id=992398 which was impossible to work around, simply stopping rendering the page after 8192px!