What does it mean to be together: what does two classes in a CSS rule mean?

Like what:
. c1.c2
{
Text-decoration:underline;
}
In the selector. C1 and. C2 directly together, no space in the middle, no commas, what does that mean? Look at an example:

This is Red

This is Italic

Here are red, italic, underline

The result has been directly described through the text, the original. C1.C2 represents the response to this style rule if the two classes are used in one label, regardless of the order, regardless of whether or not there are other classes.
However, in IE, the second paragraph, in addition to the italic and underline, shows that it also regards. C1.c2 as a. C2.
So, for compatibility reasons, try not to use a selector like C1.C2.
This article links http://www.cxybl.com/html/wyzz/CSS/20120601/27374.html