Incorrect interpolation of CSS declarations into inline styles

[loilo]

Describe the bug
Inline styles with interpolations inside can behave very strangely. This only seems to be the case when a whole CSS property name is interpolated.

This is very hard to describe with words and I probably haven't even grasped the exact cause for this error, so please take a look at the comprehensive REPL linked below.

To Reproduce
Here's the REPL. I've tried to boil it down as much as I could and cover any edge cases I could imagine.

Severity
There are workarounds to avoid this issue, so now that I know it, this is just a minor annoyance for me. However it is completely unexpected and incorrect behavior so it will hit users harder who are not as experienced with debugging issues like this.

[Conduitry]

This looks like a duplicate of #1830. I'm not entirely sure which of these we're going to consider bugs, but it's good to have a list of some more weird cases. I'll add this to the other issue.

[loilo]

Aw, saw that issue during preparation but did not realize it's actually the same cause. Sorry for not spotting the duplicate.

[Rich-Harris]

Fixed in 3.9.0! https://svelte.dev/repl/5d68597ce6854beba658aed433632f22?version=3.9.0