ASP.NET - CSS - How to apply 2 classes to control

Wath this example on YouTube.

Example that shows how to apply multiple css classes to one div
I have 2 classes in my css

.DefaultFontColor {

color: red;

}
 .DefaultFont {

font-size: 20px;




}

then in html
<div class="DefaultFont DefaultFontColor">


some text  </div>