Just in case someone stumbles upon this like I did and doesn't realise, the two variations above are for different use cases.
The following:
.blue-border, .background {
border: 1px solid #00f;
background: #fff;
}
is for when you want to add styles to elements that have either the blue-border or background class, for example:
<div class="blue-border">Hello</div>
<div class="background">World</div>
<div class="blue-border background">!</div>
would all get a blue border and white background applied to them.
However, the accepted answer is different.
.blue-border.background {
border: 1px solid #00f;
background: #fff;
}
This applies the styles to elements that have both classes so in this example only the <div> with both classes should get the styles applied (in browsers that interpret the CSS properly):
<div class="blue-border">Hello</div>
<div class="background">World</div>
<div class="blue-border background">!</div>
So basically think of it like this, comma separating applies to elements with one class OR another class and dot separating applies to elements with one class AND another class.