document breaking change

Author: Rich-Harris

sites/svelte-5-preview/src/routes/docs/content/03-appendix/02-breaking-changes.md

@@ -82,6 +82,16 @@ Previously, a compound selector starting with a global modifier which has univer
 
 Previously Svelte would always insert the CSS hash last. This is no longer guaranteed in Svelte 5. This is only breaking if you [have very weird css selectors](https://stackoverflow.com/questions/15670631/does-the-order-of-classes-listed-on-an-item-affect-the-css).
 
+### Scoped CSS uses :where(...)
+
+To avoid issues caused by unpredictable specificity changes, scoped CSS selectors now use `:where(.svelte-xyz123)` selector modifiers alongside `.svelte-xyz123` (where `xyz123` is, as previously, a hash of the `<style>` contents). You can read more detail [here](https://github.com/sveltejs/svelte/pull/10443).
+
+In the event that you need to support ancient browsers that don't implement `:where`, you can manually alter the emitted CSS, at the cost of unpredictable specificity changes:
+
+```js
+css = css.replace(/:where\((.+?)\)/, '$1');
+```
+
 ### Renames of various error/warning codes
 
 Various error and warning codes have been renamed slightly.