The default WTP plugin bundled with the Eclipse installation doesn't support this. You will have to install an extension plugin (WTP Web resources).
Go to Help -> Install New Software and add the following URL in the 'Work with' text box and press Add.
http://oss.opensagres.fr/eclipse-wtp-webresources/1.1.0/
After a while, you'll be presented with a check list. Check the plugin and click Install.
After the installation is complete, restart Eclipse. Then open up an HTML file and add a link to a CSS file like in the following example:
sample.html
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<link rel="stylesheet" type="text/css" href="test.css" />
</head>
<body >
</body>
</html>
sample.css
.one { padding: 10px; }
.two { padding: 20px; }
Now add an element within the body of the HTML file - e.g. div - and add a class="" attribute. Pressing Ctrl+Space within the double quotes will bring up the content assist window showing you all the available classes from the linked CSS file.