PROPOSAL:

  Update the definition of the content of the @longdesc attribute to match the requirements for the @src attribute.


CURRENT SITUATION:

  Referring to its definition in HTML5,[1] the HTML Image Description Extension spec currently says that the content of the @longdesc attribute MUST be:

   ]] a valid URL potentially surrounded by spaces [[

  This definition, however, is too loose. For contrast, the definition of the img element’s @src attribute pointes to HTML5’s definition of 'a validt non-empty URL'.[2] Thus, @src is required to be: [3]

   ]] a valid non-empty URL potentially surrounded by spaces [[


WHY CURRENT SITUATION IS DANGEROUS:

  The consequence of the above is that the HTML5 validator whines if the @src attribute only contains the empty string (including the empty string surrounded by spaces) - which is nice. But, conversely, if the @longdesc attribute contains the empty string (potentially surrounded by spaces), the validator does (currently) not whine.[4] 

  It is also worth mentioning that in the infamous Longdesc Lottery, then "is blank" (followed by "is not a valid URL"), is the first mentioned example of longdesc misuse.[5] When possible, the HTML Image Description Extension spec should demonstrate that it tries to avoid the issues that the Longdesc Lottery article takes up.

[1] http://www.w3.org/html/wg/drafts/html/master/infrastructure.html#valid-url-potentially-surrounded-by-spaces
[2] http://www.w3.org/html/wg/drafts/html/master/infrastructure.html#valid-non-empty-url
[3] http://www.w3.org/TR/html5/embedded-content-0.html#attr-img-src
[4] http://tinyurl.com/emptyLongdesc
[5] http://blog.whatwg.org/the-longdesc-lottery