Styles applied to this page:

p.test1:first-letter {color: green; font-size: 200%;}
p.test1 {color: gray; font-size: 1em;}
p.test2 {color: gray; font-size: 1em;}
p.test2:first-letter {color: green; font-size: 200%;}

This paragraph has a class of test1, and its text should be gray. This should apply to the first letter, whose rule has a specificity equal to the p.test rule.

This paragraph has a class of test2, and its text should be gray. This should apply not to the first letter, whose rule has a specificity equal to the p.test rule, and which comes later in the stylesheet.