Add the rule for the relationship between srcset, sizes, and loading attributes.

[YusukeHirao]

Cite: https://html.spec.whatwg.org/multipage/images.html#srcset-attributes

Valid the descriptor in srcset

If an element has a sizes attribute present, all image candidate strings for that element must have the width descriptor specified.

sizes=auto on lazy state

The auto keyword may be specified in the sizes attribute of source elements and sizes attribute of img elements, if the following conditions are met. Otherwise, auto must not be specified.

The element is a source element with a following sibling img element.

The element is an img element.

The img element referenced in either condition above has a loading attribute which is in the Lazy state.