If you get an error like this

Symbol's function definition is void: apropos-macrop

When editing CSS files with Emacs on Debian (testing or unstable at the time of writing), then this is because the package css-mode includes code that no longer works with Emacs version 24 and has been abandoned (emacs contains its own css-mode).

The solution is simply to uninstall css-mode:

sudo apt-get remove css-mode

That’s it.