What is the meaning of spaces in CSS selectors and the difference between .classA.classB and .classA .classB?

Question

What is the difference between these two selectors?

.classA.classB {
  border: 1px solid;
}

.classA .classB {
  border: 1px solid;
}

Answer 1

.classA.classB refers to elements with both class A and class B (class="classA classB"); while .classA .classB refers to the descendant element of an element with class="classA", and the descendant element has class="classB".

Edit: Reference specification: Attribute selectors (see Section 5.8.3 Class Selectors)