Support <img sizes=auto> and <source sizes=auto>

[zcorpan]

URL of WHATWG spec or PR: https://github.com/whatwg/html/pull/8008

Describe the solution you'd like Support checking sizes=auto or omitted sizes attribute as specified.

Valid:

<img alt loading=lazy srcset="a 100w" src=a>
<img alt loading=lazy srcset="a 100w" src=a sizes=auto>
<img alt loading=lazy srcset="a 100w" src=a sizes=AUTO>
<img alt loading=lazy srcset="a 100w" src=a sizes="auto, auto">
<img alt loading=lazy srcset="a 100w" src=a sizes="auto, 100px">
<img alt loading=lazy srcset="a 100w" src=a sizes="auto, (min-width: 100px) 100px">
<img alt loading=lazy srcset="a 100w" src=a sizes="(min-width: 100px) 100px, auto">
<img alt loading=lazy srcset="a 100w" src=a sizes=auto width=0>
<img alt loading=lazy srcset="a 100w" src=a sizes=auto style="width: 0px">

(The last two could maybe trigger a warning, since it will be treated as 100vw.)

Invalid:

<img alt srcset="a 100w" src=a>
<img alt srcset="a 100w" src=a sizes=auto>
<img alt loading=eager srcset="a 100w" src=a>
<img alt loading=eager srcset="a 100w" src=a sizes=auto>
<img alt loading=lazy src=a sizes=auto>
<img alt loading=lazy  srcset="a 2x" src=a sizes=auto>
<img alt loading=lazy srcset="a 100w" src=a sizes="100px, auto">

Similarly for <picture><source ...><img ...></picture>

<link rel=preload as=image imagesrcset="a 100w" href=a imagesizes="auto"> is invalid. (Also see #826)

[zcorpan]

Also see https://github.com/whatwg/html/pull/9493