[Zalastax]

It looks like fairly standard JS to me. I don't know where to read about it but there are some details on the deprecation blog article: https://blogs.msdn.microsoft.com/ie/2008/10/16/ending-expres...

[richardwhiuk]

That's CSS, with embedded JS, which is relatively non standard.

[cstejerean]

It’s very non-standard, it was IE only until they dropped it in IE8, but it was one of the features being abused to make up for other css features that IE6 or 7 lacked.

[Zalastax]

Sure, but my point still stands: figuring this out doesn't require arcane knowledge, contrary to what was suggested. The code inside the expression looks like standard JS. The downvotes and your reply suggests I should have communicated more clearly.