As far as I know, Slimjet Linux does not support proprietary HTML5 codecs such as H.264 yet, unlike the Windows builds.
This is the only reason why Slimjet is still not my main browser on Linux. Because otherwise I very often have to switch between Slimjet and another, namely every time I encounter a video which Slimjet cannot play but the other does.
So I'm longing for the day when Slimjet fully supports HTML5 media codecs.