[Simon]

Rockwell is a (geometric slab) serif design,
Trebuchet MS is a sans-serif typeface,
Verdana is a sans-serif typeface.

From a design standpoint, you really want to pick either serif or sans-serif for your font-families. For serif fonts, you'd be safe with Georgia and Times New Roman. Also, as I mentioned before, you want to stick either "serif" or "sans-serif" at the end of all font-family choices in your stylesheet. You have that in one place but not in the others.

Remember, when you use a font that has more than one word, enclose the name in quotes, like this example:

font-family: Rockwell, Georgia, "Times New Roman", serif;

Note that the comma goes outside the quotes in this usage.