When there is a space between two selectors, it represents a subclass selector

.a .b{}
Copy the code

Represents the B subclass of class A

Without Spaces between the two selectors, they represent labels with both class names

<div class="a b"></div>
.a.b{}
Copy the code

When there is a comma between two selectors, it means that two class selectors are selected

<div class="a"></div>
<div class="b"></div>
.a,.b{}
Copy the code