mockup web layout font is smaller than illustrator

Question

It may sound obvious:

I'm used to design mockups of my webpages with illustrator.When I translate everything to css and test the page into my browser I can see that fonts are bigger in the illustrator mockup even if I used the same font setting on the css side.

//in css i have
body{font:Georgia,"Times New Roman",Times,serif;font-size:16px;
font-weight:normal;}

//I wrapped the example text inside "h1" tags 
h1{font-size:24px;font-weight:normal;}

//in illustrator I have font georgia,24px,font-style "regular" selected

How could I fix that?

thanks

Luca