如何一次将样式应用于多个类？

=> 要么
 
.abc, .xyz {
margin-left:20px;
width: 100px;
height: 100px;
} 
 
=> 要么
 
a.abc, a.xyz {
margin-left:20px;
width: 100px;
height: 100px;
} 
 
=> 要么
 
a {
margin-left:20px;
width: 100px;
height: 100px;
}