The following text in HTML-AAM:

~If "any labeling attribute … specifies an empty label … indicates the author's intention to indicate that the img is decorative or redundant. In this case, the user agent MUST set the name to an empty string."

Conflicts with WCAG Success Criteria H67: https://www.w3.org/TR/WCAG20-TECHS/H67.html

For each image that should be ignored: Check that title attribute is either absent or empty. Check that alt attribute is present and empty. ...

Furthermore, both Blink and Gecko allow this behavior. WebKit currently matches the HTML-AAM spec, but will likely change to match the other behavior. https://webkit.org/b/173870

Test case 7 here:
https://bugs.webkit.org/attachment.cgi?id=314203