Check out example codes for "select two stuffes css". It will help you in understanding the concepts better.

Code Example 1

//HTML Code
<div class = 'firstClass secondClass'></div>

//CSS Code
.firstClass.secondClass {
  
}

Code Example 2

#load p,
#load h1,
#load h2,
#load h3,
#load h4,
#load h5,
#load h6 {
  font-size: 80%;
  margin:2%;
}

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