criticalCSSRuleType.js

styleSheets.map((sheet) =>
    Array.from(sheet.cssRules).map((rule) =>
      rule.type === rule.FONT_FACE_RULE
        ? { /* include this rule */ }
        : rule.type === rule.STYLE_RULE
        ? { /* include this rule if above-the-fold */ }
        : rule.type === rule.MEDIA_RULE
        ? Array.from(rule.cssRules)
            .map((mediaRule) =>
              mediaRule.type === mediaRule.FONT_FACE_RULE
                ? { /* include this rule */ }
                : mediaRule.type === mediaRule.STYLE_RULE
                ? { /* include this rule if above-the-fold */ }
                : { /* don't include */ }
            )
        : { /* don't include */ }
    )
  )