selector can be grouped to form another selector on a union level with a comma.
Example: selector a or selector b
a, b
You can make an intersection (and) on a attribute level.
On a element level, it has no sense to want to select the element A and the element B as it will always never return any node in the tree.
Example: The element E with the attribute att that has the value foo and the attribute att2 that has the value bar
E[att=foo][att2=bar]]
An union or an or is accomplished be separating the selector by a comma.
Example: .a, .b
#IdName, .className { color:blue; }
<div id="IdName">The blue color CSS style will be applied on this text</div>
<div class="className">and also on this one</div>