selector differences
HTML
<div class="A">class A</div>
<div class="B">class B</div>
<div class="A B">class A and class B</div>
<!-- same -->
<div class="D">class D</div>
<div class="E">class E</div>
CSS
.A {
background: lightblue;
}
.B {
background: pink;
}
.A.B {
background: #66cc66;
}
.D, .E {
background: #6666cc;
}