CSS parser rejects minified `:nth-child` `of` syntax (`2n of.foo`) — REGEX_NTH_OF requires whitespace on both sides of `of`

Describe the bug

REGEX_NTH_OF in src/compiler/phases/1-parse/read/style.js matches the <an+b> of <selector> form only with whitespace on both sides of of:

const REGEX_NTH_OF =
	/(even|odd|\+?(\d+|\d*n(\s*[+-]\s*\d+)?)|-\d*n(\s*\+\s*\d+))((?=\s*[,)])|\s+of\s+)/y;

But CSS minifiers (cssnano/lightningcss) legally collapse 2n of .foo → 2n of.foo — the . delim token terminates the of ident, so no whitespace is required there. Browsers accept the minified form (we serve it in production today). Svelte's parser throws css_expected_identifier on it.

Real-world impact

SvelteKit's inlineStyleThreshold re-parses the built (minified) CSS through this parser, so any component using :nth-child(... of .selector) compiles fine on its own but fails the production build the moment its emitted CSS file falls under the inline threshold:

[vite-plugin-sveltekit-compile] Expected a valid CSS identifier
https://svelte.dev/e/css_expected_identifier

Reproduction

Parser-level, no project needed:

import { parse } from 'svelte/compiler';

parse('<style>.x:nth-child(2n of .x){color:red}</style>', { modern: true }); // ok
parse('<style>.x:nth-child(2n of.x){color:red}</style>',  { modern: true }); // throws css_expected_identifier

End-to-end: a SvelteKit app with kit.inlineStyleThreshold set and a component style containing :nth-child(even of .foo) — vite build fails.

Expected

The trailing \s+ requirement after of should allow any token that lawfully starts the following <complex-selector> (., #, [, :, *), e.g. something like \s+of(?=\s|[.#\[:*]).

Severity

annoyance (workaround: keep the affected CSS file above inlineStyleThreshold, or avoid the of syntax)

System Info

• svelte 5.50.1; regex unchanged on current main
• hit via @sveltejs/kit 2.61.1 inlineStyleThreshold + vite 7 css minification