From http://www.w3schools.com (Copyright Refsnes Data)

DHTML - CSS

JavaScript and the HTML DOM can be used to change the style of any HTML element.

Change Style of the Current HTML Element

To change the style of the current HTML element, use the following statement:

this.style.property=new style

<html>
<body>

<h1 onclick="this.style.color='red'">Click Me!</h1>

</body>
</html>

To change the style of a specific HTML element, use the following statement:

document.getElementById(id).style.property=new style

<html>
<body>

<h1 id="h1" onclick="document.getElementById('h1').style.color='red'">Click Me!</h1>

</body>
</html>

For all style properties, look at our complete HTML DOM Style Object Reference.

To learn more about CSS, please read our complete CSS tutorial.

Mouse Events
Change the color of an element when the cursor moves over it.

Visibility
How to make an element invisible. Do you want the element to show or not?