New principle: Avoid exposing API that result in synchronous flushing of CSS or layout

[atanassov]

This was brought up in our discussion of exposing a new Range API..

During review of the issue, prior art of similar API, and guidance we have written our conclusions is that we need a new design principle. This is also closely related to the issue or better defining the HTML loop and what causes bucketing and flushing. At the very least we need to specify what forces layout or CSS to flush and provide some guidance on how to avoid it.

[plinss]

See also: w3ctag/design-reviews#734