`attr()` is being updated to basically do that, the modern spec lets you specify a datatype and access any attribute (with some exceptions, you can't get URL types for security reasons), then use it generally.

E.g: aspect-ratio: attr(width px) / attr(height px);

Interestingly, the bug for this feature is already 17 years old (https://bugzilla.mozilla.org/show_bug.cgi?id=435426).