[CSS-images] Specify fallback behavior when replaced or background image content not available

[mozilla-apprentice]

A resolution was made for csswg-drafts/#1984.

[CSS-images] Specify fallback behavior when replaced or background image content not available

• RESOLVED: Loading images are not invalid but treated similarly as explained in https://drafts.csswg.org/css\-images\-3/\#image\-values

Discussion.

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[emilio]

I think this matches our implementation. https://drafts.csswg.org/css-images-3/#invalid-image