Fail the gate on a stylesheet feature the browser floor lacks, unless an @supports guards it

README.md

@@ -1059,9 +1059,11 @@ Chromium 87, Safari 15 and Firefox 115, the engines in years-old TVs, kiosks and
 features [Baseline Widely available](https://web-platform-dx.github.io/baseline/) (in every
 major engine for 30 months) may shape how a page looks; anything newer, like popovers or anchor
 positioning, only enhances. A feature the floor engines lack, such as `:has()`, may
-decorate a page but never decide whether something shows or can be read. A colour is hex, `rgb()` or
-`hsl()`, never `oklch()`: a custom property holding it passes unchecked, and the `var()` reading it
-then renders transparent.
+decorate a page but never decide whether something shows or can be read. The tests hold every
+stylesheet in `public/` and `examples/` to this floor, against MDN's compat data: a feature a floor
+engine lacks fails them unless it sits inside an `@supports` whose condition names it
+(`src/ui/stylesheet-floor.ts` lists what it checks). A plugin stylesheet mounted from outside the
+repo is not checked.
 
 **A field whose options depend on another field is asked for in steps, one GET form each**, so every
 choice is in the URL, and nothing typed is lost to one because nothing is typed until they are made.