CSS custom properties handled as errors

[nosovk]

CSS custom properties introduced in sveltejs/rfcs#13 currently broke eslint cheking.

<input --width="{numLength * 0.875 + 3}rem">

Will fire: '--width' is not a valid attribute name ValidationError'

[dummdidumm]

Custom properties only work on components, not elements, so this is kind of expected.

[Conduitry]

Yeah, this isn't a valid Svelte component. That's an error from the compiler that you can also see if you paste that into the REPL.