Grouping

When several selectors share the same declarations, they may be grouped into a comma-separated list.

Example(s):

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 }

CSS offers other "shorthand" mechanisms as well, including multiple declarations and shorthand properties.