Any web monkeys out there?

I'm using:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 TRANSITIONAL//EN" "http://www.w3c.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

I have the following in my css file:
p.med_web {
  font-size: 150%;


The following works well.
<p class="med_web">blah blah blah</p>

Why doesn't
<span class="med_web">blah blah blah</span>

work???

Thanks,

Mark Jarvis