CSS3 does not support any notion of mixins.
That is only available using a CSS compiler like SASS or LESS. (That's kind of the whole point of using compiled CSS... if CSS had it built in, we wouldn't need the compilers.)
Question
I have been looking around how to use mixins with regular css3, no sass or less, so I was wondering if that was possible. All I want to do is:
@mixin .clientred{
color:darkred;
}
in a nutshell so then I could use it later
h1{
include: .clientred;
font-size: 32px;
}