css class definition with multiple identifiers

In CSS with the .className selector you can define the properties for every element with “className” class. Every element could have more classes.
The meaning of a selector with more classes depends on how you combine them in your declarations:

• .class1.class2 will match only the elements that have both of them classes defined.

  .class1.class2 { background: red; }
  <div class="class1 class2"></div>
  

• .class1, .class2 will match the elements with .class1 or .class2

  .class1, .class2 { background: yellow; }
  <div class="class1"></div>
  <div class="class2"></div>
  

• .class1 .class2 will match only the elements with class2 within elements with class1.

  .class1 .class2 { background: blue; }
  <div class="class1">
      <div class="class2"></div>
  </div>