The PSL nor its volunteers prescribe what browsers do with entries or their recognition/handling of entries, default behaviours, or other handling. Browsers do what browsers do, diversely, and the PSL is not the boss of them.

But more so, external style cannot be applied to a subsection of a web page unless they force it into an iframe, which has all sorts of issues of it's own which is why external CSS is usually ignored. Inline CSS is often stripped by the tag strippers who don't want you turning things on or off... and media queries shouldn't even play into it since the layout should be controlled by the page it's being shown inside (for webmail) or the client itself, NOT your mail.

putting a line break before the directive in the Gem (not suitable because I don't own the gem)

There's not much we can do there. It's not possible for a Svelte component to inspect another Svelte component and check if it exposes any prop

I understand this is not ideal, but sadly this is not something we can change as it's how Svelte works.

Just coming here to voice my agreement that these warnings are annoying and exist in other libraries as well. For me this happened with svelma. I didn't write the library code, so I don't have complete control over it even though I agree there is an argument to be had around whether I should be notified anyway. In either case, these warnings should be easily disabled since libraries don't always get updated over night.