[css4-images] possible error in <image-set-decl> syntax

should <image-set-decl> be

<image-set-decl> = [ <image>
<http://dev.w3.org/csswg/css4-images/#image-type> | <string> ]
<resolution>

or

<image-set-decl> = ( <image>
<http://dev.w3.org/csswg/css4-images/#image-type> | <string> )
<resolution>


in other words, can one merely specify a <resolution> without <image> or
<string>? if so, then the prose should define what this means

also, in regard to

Every <image-set-decl><http://dev.w3.org/csswg/css4-images/#image-set-decl-type>
in
a given ‘image-set()’ must have a different <resolution>, or else the
function is invalid.

what is the definition of 'different'? e.g., is 1dppx different from 96dpi?