Draft of the new CSS Selectors Level 4 spec from the W3C!
CSS control of the appearance of images that can't load
Guide to the upcoming CSS grids spec, looks super useful
Basic CSS reference
Describes a very accessible way to get started building a pattern library, and a shared vocabulary for talking about its elements.
Like CanIUse but for web type
The resources and tools for learning about the practice of front-end development. Written by Cody Lindley, sponsored by Frontend Masters
A live style guide is a great tool, but after it's been in use for a while, as the applications that use it grow, certain pain points appear. The style guide becomes unmaintainable or ignored. Lonely Planet has faced these obstacles and talks about a way forward, to a live pattern library.