Apply More than one class on element(tag)

Your Html code 

<div class="red border box">
 </div>
 
 <div class="blue border box">
 </div>
 
 <div class="green border box">
 </div>
 
 <div class="red box">
 </div>
 
 <div class="blue box">
 </div>
 
 <div class="green box">
</div>

Your css :
           
               

.box {
 width: 100px;
 height: 100px;
 margin: 10px;
 float: left;
}
.red {
 background-color: red;
}
.blue {
 background-color: blue;
}
.green {
 background-color: green;
}
.border {
 border: 3px solid black;
}