CSSSelector

4.1. Selector Lists

https://www.w3.org/TR/selectors/#grouping

A comma-separated list of selectors represents the union of all elements selected by each of the individual selectors in the selector list. (A comma is U+002C.) For example, in CSS when several selectors share the same declarations, they may be grouped into a comma-separated list. White space may appear before and/or after the comma.

CSS example: In this example, we condense three rules with identical declarations into one. Thus,

 h1 { font-family sans-serif }
 h2 { font-family: sans-serif }
 h3 { font-family: sans-serif }

Is equivalent to:

 h1, h2, h3 { font-family sans-serif }