Change the font size :
Change the active stylesheet : red blue green
Change the class of one element : sans serif

change styles

This page demonstrates JavasScript that changes the appearance interactively.

First, the two buttons change the font size by altering a font-size property directly.

Second, the three radio buttons enable one CSS file and disable two others.

And third, the checkbox changes the class of the first h1 tag.

All of these effects are implemented here with 'onclick' handlers on various controls in a form; however, they could just as easily happen in reponse to any sort of event on any sort of element, including mousing over an image, clicking on a link, or whatever.