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.
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.