Describe the bug
Purgecss purges :where() top level rules from the astro build.

To Reproduce
Steps to reproduce the behavior:

Write this in your css:

:where(h1, h2, h3) {
  border: 1px solid red;
}

Then npm run build in your Astro project. Inspect the bundled CSS file and the rule will be gone.

One potentially helpful detail, if your :where() is not at the top level, then it won't be purged, so something like this for instance will work as expected:

body :where(h1, h2, h3) {
  border: 1px solid red;
}