IE also had a custom image filtering scripting language embedded in CSS that was pretty dangerous, you could easily hang someone's whole computer if you weren't careful. A common hack to find was adding png transparency which IE infamously lacked, at great computational cost to the user (this was at a time where the lack of other CSS features made PNG transparency quite necessary for building fancy UIs).

It's worth pointing out that many of the old ugly things were IE specific including the feature referenced by the parent comment... it was a time when "standard" had little weight, although some of the good ideas like box-sizing were adopted by W3C.

Today we have CSS custom properties which do similar things:

- Authors can define custom properties as long as they start with a double-dash, like --demo

- Authors can leave these values empty, or put any code that doesn't break CSS's syntax inside. This can be CSS values, it can be JSON, it can be other languages, even some JavaScript could be stored in CSS to be read later

- Using JavaScript you can read the values of CSS custom properties, and set them

So it's possible with a tiny bit of JS to re-create what the old IE expression() function used to do in any modern browser in a fully standards-compliant way