Forward to the 16:00 mark to get a good overview of how browsers go from receiving a tad of CSS to actual pixels on screen, a process known as the rendering pipeline:
- Recalc styles
- Composite Layers
If you like reading more than watching a video, a write-up is also available: Pixels are expensive →
Note: If you’ve been reading the posts here on bram.us tagged performance, you’ll most likely already know this stuff. It’s a good refresher nonetheless.