In reply to Matt.

We're not specifically talking about a block stylesheet. But, that is actually a good example where core has made it easier to opt out of something, which is very much a good thing. I would like to see that level of granularity with any design-related feature.

The primary example in this post outputs inline styles for all blocks. It's a global top margin, and there was no way to remove it. This would force themers into a specificity battle and bloat CSS. Fortunately, they have actually made it an opt-in feature with the goal of eventually turning it on by default but allowing theme authors to opt out.

But, the crux of the issue is that it was intentionally made to not be overwritable from the outset.