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