[tracking] A list of nowadays good practices

[sole]

Let's collect 'good practices' that WebRenderer turns into trivial solved issues so we can demonstrate its performance even if using 'bad practices'

Sam suggested https://developers.google.com/web/fundamentals/performance/rendering/ as a good collection of techniques for avoiding jank.

[sole]

A description of how Layout is calculated in Servo: https://github.com/servo/servo/wiki/Layout-Overview