The space between the comma and the selector css

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

.a .b{}

B represents a subclass of the class

And under no circumstances space between the two selectors, it represents the label also has two class name

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

When the selection is between two commas, the representative of the two is selected class selector

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