`<style scoped>`
However, it was abandoned partly because it "adds a lot of complexity to how the scoped style system works for Shadow DOM".
https://chromestatus.com/feature/5374137958662144
https://groups.google.com/a/chromium.org/g/blink-dev/c/R1x18...
There were other reasons at the time as well, as mentioned in the linked email: the feature wasn't owned, wasn't shipping in any browser, and was making it difficult to improve the Shadow DOM styling system which was shipped already.
That was also a long time ago, and I think browsers and standards groups have learned a lot about designing features to be backwards compatible and how to avoid certain implementation complexities.
Component-scoped styles is a very handy feature. It could be achieved just by leveraging selector specificity, but that's rather complex and a lot of people hate CSS so less cognitive overhead is better.
This is why I love Vue. It's basically the nightly version of HTML/CSS/JS. The syntax from this article is already familiar from Vue. It extends web standards, rather than replacing them, IMO.
I've seen it work on Chrome, Firefox and Edge. But at the time I discovered it, Chrome had already scheduled it for exclusion.
It was shipping in Firefox when I removed it from Chrome. It was moved behind a flag in Firefox shortly after and then support was removed.