[css-sizing-4] aspect-ratio and <picture>

[cbiesinger]

https://drafts.csswg.org/css-sizing-4/#aspect-ratio

How should aspect-ratio interact with <picture>?

A straightforward application of the spec would have it just use the aspect-ratio on the inner <img>. But it may be useful to specify different aspect-ratios for the different <source>s, so maybe aspect-ratio should "inherit" from the active source to the <img>?

cc @fantasai @tabatkins

[emilio]

Related: whatwg/html#4968

[fantasai]

This should be handled by HTML: HTML should be finding the effective width/height values for the effective image source (probably drawing from the width/height attrs on the chosen <source>), and map those into an aspect-ratio: x / y auto declaration at the HTML Preshint cascade level.