[whatwg] <link rel=icon sizes=?> What if sizes is incorrect?

[Ernest Cline]

On further reflection, I'll concede that the style attribute is probably better suited to deciding what to do with the icon once it is fetched. Using it as metadata to decide what is fetched is problematic if multiple sizes are to be allowed to be specified in a single link element.  However I do see some problems with the proposed sizes attribute.

Mainly, I am troubled by the statement:
  "The keywords specified on the sizes attribute must not represent icon sizes that are not actually available in the linked resource."

No matter what the spec says, I think we can all agree that once this enters the real world there will be times when the icon size given is wrong.  So what to do?  At a minimum, I think the behavior called for when dealing with the type attribute is called for: 
  "User agents must not consider the type attribute authoritative — upon fetching the resource, user agents must not use metadata included in the link to the resource to determine its type."

However once a mismatch is discovered, does that mean that the icon that has been retrieved should be scaled as if it were an <img>, or should the icon be discarded and the next icon which matches the available parameters be used instead?