You need to use a scalable font size rather than pixels (Internet Explorer doesn't scale pixels). The em unit is best.
The default font size in most browsers is 16px. So 1em is equivalent of 16px.
For easy use of ems, set the font size on the body element to 0.625em so that 1em will be roughly equal to 10px (because 10/16 = 0.625).
body {
font-size: 0.625em;
}
Now, for example, if you want an html element to be 15 pixels, you set its size to 1.5em.
