In your CSS, you specify that your #header, #footer and #sidebar elements should be baby blue. A week later, your picky designer asks that they be changed to lilac. Of course, you can change the color for all three elements, but, as the DRY gods will tell you, find/replace is error-prone.
You should only specify colors (and, preferably, most attributes) only once in your stylesheet.
The Wrong(ish) Way:
.header {
color:baby-blue;
font-size:humungo
} .footer {
color:baby-blue;
font-size:tiny;
}
The Right(er) Way:
.header, .footer {
color:baby-blue;
} .header {
font-size:humungo
} .footer {
font-size:tiny;
}