CSS Class Highlighter

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export function readableRules(element: Element) {
  const root = element.getRootNode() as Document | ShadowRoot;
  const sheets = new Set<CSSStyleSheet>([
    ...('styleSheets' in root ? Array.from(root.styleSheets) : []),
    ...Array.from(
      root.querySelectorAll<HTMLStyleElement | HTMLLinkElement>('style, link[rel="stylesheet"]'),
    ).flatMap((node) => (node.sheet ? [node.sheet] : [])),
    ...(root.adoptedStyleSheets ?? []),
  ]);
  const rules: MatchedRule[] = [];
  let unavailable = 0;
  let visited = 0;
  const walk = (list: CSSRuleList, source: string, condition = '') => {
    for (const rule of Array.from(list)) {
      if (++visited > 6000 || rules.length >= 200) return;
      if ('selectorText' in rule && 'style' in rule) {
        const styleRule = rule as CSSStyleRule;
        try {
          if (element.matches(styleRule.selectorText))
            rules.push({
              selector: styleRule.selectorText,
              css: styleRule.style.cssText,
              source,
              condition,
            });
        } catch {
          /* Pseudo-elements and nested selectors need DevTools cascade inspection. */
        }
        // …