CSS Selector list invalidation

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。