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;
}