We need to provide a smooth way to transition to nesting, so authors can detect whether nesting is supported by the user agent. So they can provide fallback solutions for UAs that don't support it.

There where already some prior discussions related to this topic, though no explicit issue or resolution regarding this, as far as I can tell.

The major discussion related to this is #8349, but its focus is rather on error recovery than feature detection. Another slightly related one is #2463, though it rather features detection of at-rules and their descriptors.

So I opened this issue to explicitly discuss how nesting support can be made detectable. I believe this is a must for any implementation for shipping nesting capabilities to reach a high level of adoption and avoid page breakages.

Sebastian