Mixins and functions might be of some use if the end-user (rather than the document author) is able to use them to override the meaning of existing CSS commands

Mixins and functions might be of some use if the end-user (rather than the document author) is able to use them to override the meaning of existing CSS commands. That still isn't the best way of doing such a thing, though; but it would help a bit. (If done well, it might also allow extensions to implement some parts of CSS in CSS, therefore allowing browser writers to omit some stuff.) (However, it is unfortunate that WWW is so messy that such things are even necessary)