Multiple classes for an element

To give an element multiple classes, separate the classes with spaces:
[color #cc6600]<div class="important news">...</div>
<div class="important post">...</div>[/color]

Why? It can make CSS more efficient:
[color #cc6600]<style>
div.news{border:1px solid orange;padding:.5em;background:#cdc;}
div.post{border:1px dotted black;padding:.5em;background:#eee;}
.important{font:bold 120% sans-serif;color:red}
</style>[/color]

For reference, the corresponding inefficient CSS might look like this:
[color #cc6600]<div class="importantnews">...</div>
<div class="importantpost">...</div>[/color]

...

[color #cc6600]<style>
div.news{border:1px solid orange;padding:.5em;background:#cdc;}
div.post{border:1px dotted black;padding:.5em;background:#eee;}
div.importantnews{border:1px solid orange;padding:.5em;background:#cdc; font:bold 120% sans-serif;color:red}
div.importantpost{border:1px dotted black;padding:.5em;background:#eee; font:bold 120% sans-serif;color:red}
.important{font:bold 120% sans-serif;color:red}
</style>[/color]

This technique is stated in the W3C spec:
http://www.w3.org/TR/html4/struct/global.html#h-7.5.2

Supporting browsers reportedly include, but are not limited to:
IE 5.0, 5.5 and 6, Mozilla 1.4 and 1.5 and Firebird 0.7, Opera 7, IE for Mac and Safari for Mac.