We can force certain language upon a code block by adding such a line above the code:
After quick trial and error, this one did the trick:
More details can be found here.
I disagree that conditional comments should be handled by the default syntax highlighter. For one thing, they are an ancient relic, and even IE is discontinuing support for it – as the page you linked to says:
Fun fact: Until 2012 (!), that page contained the following text instead:
More importantly however, a conditional comment is just that – a comment. So in the context of HTML, it is correct for the syntax highlighter to highlight it as such.