Function validators

[delorge]

There is a bunch of css functions that don't have appropriate validator yet. This issue is created in order to keep track of progress in this kind of functionality.

• [x] calc
• [x] clip-path
• [x] cross-fade
• [x] element
• [x] filter
• [x] minmax
• [ ] transform
• [ ] attr
• [x] gradient (proprietary support currently outstanding)

Feel free to update this post (or point out missing functions)