`:not` selector doesn't work with class-directive

[sockmaster27]

Describe the bug

For the following Svelte component I would expect that the h1 element would be styled with a red background:

<script>
	const x = false;
</script>

<h1 class:x>Hello, world!</h1>

<style>
	h1:not(.x) {
		background-color: red;
	}
</style>

Instead, no background color is applied to the element and a warning is produced:
Unused CSS selector "h1:not(.x)"

Reproduction

https://svelte.dev/playground/7e9220bafc094e209bb9a53e7e7bb48d?version=5.1.10

Logs

No response

System Info

N/A

Severity

annoyance

[Rich-Harris]

I came to the issue tracker to report the same thing — this works in 5.1.9, not in 5.1.10. It broke in #13999

[Conduitry]

See also #14168, another thing that was broken.

See also also #14166, something that (I think?) was fixed.

[Rich-Harris]

closing as dupe of #14168