@position-try
The rules that tell the browser what to do when an anchor-positioned element doesn't fit where it was supposed to go.
A tooltip placed below its button works perfectly until the button happens to sit at the bottom of the screen, at which point the tooltip needs to go above instead. That is precisely the problem libraries like Floating UI and earlier Popper.js existed to solve, with measurements and recalculations in JavaScript on every scroll. With position-try-fallbacks you instead give an ordered list of alternatives the browser tries in turn until one fits — either ready-made keywords such as flip-block and flip-inline, or your own @position-try blocks where any properties can be rewritten for that alternative. Available in Chromium since 2024; other engines are behind.