MDN 里有这么一段: A downside to using selector lists is that the following aren’t equivalent: h1 { font-family: sans-serif } h2:maybe-unsupported { font-family: sans-serif } h3 { font-family: sans-serif } h1, h2:maybe-unsupported, h3 { font-family: sans-serif } This is because a single unsupported selector in a selector list invalidates the whole rule. 这在 W3C 里也有定义，见 4.1. Selector Lists 解决方案 MDN 也提出一个变通办法，用 :is()。 A way to remedy this us to use the :is() selector, which simply ignores invalid selectors in its arguments, but at the cost of all selectors having the same specificity, because of how :is() calculates specificity. :is(h1, h2:maybe-unsupported, h3) { font-family: sans-serif } 不过这个功能没在 W3C 里定义，应该是浏览器自己的实现。