Detecting at-rule support in CSS with @supports at-rule(@keyword)

CSS is gaining a new feature detection capability via `@supports at-rule(@keyword)`, shipping in Chromium 148. It lets developers check whether a browser supports a specific CSS at-rule (like `@starting-style` or `@view-transition`) before using it. The feature can also be used in `@import` supports conditions. Notably, the originally proposed extensions for detecting specific descriptors, preludes, or full at-rule blocks have been dropped from the spec. Firefox and Safari do not yet support this feature. Self-detection is possible by checking `@supports at-rule(@supports)`.