One of the most desired interactions on a Web page is to have something change when the reader moves his or her mouse over it. This is called a mouseover event. With Dynamic HTML this effect is very easy to produce. Every element on a Web page has a style associated with it. Even if you didn't declare a style, the browser has a default style for everything.
Paragraphs use the browser's default typeface.
The body background is grey. The list goes on and on, but the point is that every element on the page has a style. Stylesheets, via CSS, let you change those. The style property we want to change. Just a smidgen of simple JavaScript.