Version:           4.0 (using KDE 3.1.4)
Installed from:     (testing/unstable)
Compiler:          gcc version 3.3.2 20030908 (Debian prerelease)
OS:          Linux (i686) release 2.6.0-test3

when a line of a CSS is commented using //, that line is used anyway:
<style>
.test {
	color: black;
	//background-color: red;
}
</style>
<font class="test">test</font>

... will show the word text written in black with a red background.