New CSS principle: Describe when 'auto' is an appropriate property value

[jyasskin]

This was brought up in our discussion of w3ctag/design-reviews#1001 (comment) and w3ctag/design-reviews#1011 (comment).

It seems to be tempting for CSS feature designers to use 'auto' in more situations than it's really appropriate, and there ought to be guidance on when it is appropriate. We'll need help from CSS experts to figure out that guidance. (cc @fantasai @tabatkins) Some components seem to be:

• let the browser do what it thinks is best.
• the behavior that does a pretty good job for most pages.
• the behavior that should be the default, even if we can't make that happen for compatibility reasons.