The last type of selector is not really a selector, more a shorthand way
of giving the same style properties to different types of elements. You might
want every heading to be in a particular font, even though individual levels
will be different sizes.
This shorthand eliminates the tedium of recreating the same set of properties
several times, and the attendant risk of mistake. It also makes for very efficient
editing- if you want to change the font for each heading you need only do
it once, rather than for each individual level.
A selector group is simply a comma separated list of selectors. The selectors in the list can be class, pseudo class, ID, pseudo element and contextual selectors, as well as HTML element selectors.
For example, a selector group that assigns the same font to each type of link (see pseudo class selectors above) is:
A:link, A:visited, A:active, A:hover {font: times serif}
Selector groups are useful for ensuring that any sets of elements which you want to share the same properties in fact do so. By grouping like elements together you minimize the possibility of mistakenly assigning the wrong properties or values. It is also a useful time saver, both in initially setting the properties of an element, or when editing those properties, as repetition is minimized.