I worry about that being a bit of a Pandora's box — it throws encapsulation completely out the window. The nature of CSS custom properties is that they're inert unless the child chooses to do something with them, which wouldn't be the case for other things.

The problem with working around the current limitations of Svelte style (:global, svelte:head, external styles or various wild card selectors) is that the API is uglier, bigger, harder to explain AND it loses one of the best features of Svelte IMO - contextual style encapsulation. I can understand that CSS classes are a bit uncontrollable, but this type of blocking will just push developers to work around it and create worse solutions.

For my point of view, and I've been annoyingly consistent in this for as long as people have been asking for this feature or something like it, style encapsulation is one of the core principles of Svelte's component model and this feature fundamentally breaks that. It would be too easy for people to use this feature and it would definitely get abused removing the style safety that Svelte previously provided.

I'm personally surprised about that, given the degree to which web component advocates prioritise encapsulation — it seems like a footgun, honestly

A component should be in complete control of itself. Not only should a component's styles not leak out but other component's style should never leak in. Consider this 'Encapsulation part 2' if you will. When writing a component, you have certain guarantees that not only will the styles you write be contained within the component, but nothing from the outside should affect you either. You have a certain confidence that if it works in isolation, then it will continue to work when embedded within a complex application.

CSS encapsulation is a critical feature of single file components in Svelte; it allows you to think only about the styles that live together in a given component. Managing CSS has long been one of the more challenging aspects of building for the web; we have no desire to bring those problems back via official APIs that encourage the de-scoping of CSS. We do not wish to revisit the age of namespaced CSS selectors and required preprocessors.