How do I get the (element with the) highest CSS z-index in a document?

The question couldn't be easier. The z-index values are assigned by style=... or className, with Javascript or not. I don't think it matters. How do I find (with Javascript) the highest z-index? (The element it's used in would be nice, but not necessary.)

You can't use the (new) querySelector, because it doesn't query CSS values. Is there someway to query CSS? (Not the stylesheets, but the actual used values.)

Grazi