Use postcss warnings to unfixable bugs

[kadmil]

There are things we can't fix on css-only level. For such problems we could generate warnings with postcss API https://github.com/postcss/postcss/blob/master/docs/api.md#warning-class

Thank @ai for suggesting this. )

[luisrudge]

You mean for bugs that we can't evaluate if width or height, for example?

[kadmil]

Yep, like this. Or if we detect config which is known to cause bugs and can't fix them on css-only level https://github.com/philipwalton/flexbugs/blob/master/README.md#2-column-flex-items-set-to-align-itemscenter-overflow-their-container

[MoOx]

:+1:

[cvrebert]

Can this qualify as a flexbug for the purposes of warning about it?: https://drafts.csswg.org/css-flexbox/#item-margins

Authors should avoid using percentages in paddings or margins on flex items entirely, as they will get different behavior in different browsers.

[Edit: Ah, #12]