when declaring css styles what is the meaning of classA classB two class names with a dot in the middle and no space

Question

I know when there is a space in between, it will look for the class on the right within the class on the left.

.classA .classB {
    ...
}

However, what does it look for when there is no space?

.classA.classB {
    ...
}

Answer 1

The first example (space-separated classes) is a parent-child relationship. .classB is found inside .classA.

<div class="classA">
    <div class="classB"></div>
</div>

The second is for one element with multiple classes, like so:

<div class="classA classB"></div>

Very different situations, but both extremely useful!