[css-content] spec doesn't describe how content:<image> handling works

[dbaron]

While 3eef526b41b7 and da3321fc5996 (see also #216) updated css-content-3 to say that the 'content' property accepts <image> values, no other parts of the spec appear to have been updated.

In particular, 2.2. URI is still totally in terms of <url>.

The spec needs to describe how <image> values work, in terms of the definitions provided in css-images, at the level of detail needed to implement interoperably.

This is apparently already implemented in some engines (e.g., Blink, IE, Edge), and there's a Mozilla bug on implementing in Gecko.

[dbaron]

An example question that this spec should answer is: while we know that the width property doesn't apply to the image in content: "hello" url(foo.png) "world", does it apply to the image in some cases (e.g., if the image is alone (might be a compat risk if it's after a comma), or only if the image is before a comma)?

Also, I realize that perhaps section 2.2 is about the <content-list> part of the definition, and maybe that should remain about <url>. However, the handling of <image> values still needs to be defined.

[tabatkins]

Should be done. I explicitly separated out the "single image" case from the "content list" case, and defined each of the cases in greater detail.