[css21][css-grid][css-flexbox][css-align] Baselines of empty boxes

[fantasai]

Afaict, the CSS2.1 spec does not say what to do with the baseline alignment of empty inline tables. IE and Chrome use the bottom margin edge. Firefox uses the bottom border edge.
http://software.hixie.ch/utilities/js/live-dom-viewer/?%3C!DOCTYPE%20html%3E%0A%3Cdiv%3ETest%20%3Cdiv%20style%3D%22display%3A%20inline-table%3B%20background%3A%20blue%3B%20width%3A%2010px%3B%20height%3A%2010px%3B%20margin%3A%2010px%3B%20border%3A%20solid%20orange%22%3E%3C%2Fdiv%3E

The Grid and Flexbox specs currently say to use the bottom content edge for empty grid/flex containers.

We should spec something consistent for all three.

[fantasai]

Half-baked Proposal: State that such boxes have no baseline, and one is synthesized by the context--using the margin box for inline baseline-alignment, and the border box (as we just resolved) for flex/grid baseline-alignment.

[atanassov]

CSSWG Resolution: Have grid and flexbox keep saying that empty boxes have no baseline and they're synthesized with bottom border edge.

https://lists.w3.org/Archives/Public/www-style/2016Sep/0028.html