There has been a fair bit of bad press about the non-validation of the code. It pertains to anyone using an SEO system which usually requires XHTML to validate.
I have gone through the code for you on various sites that use the mod and the general errors appear to be.
Error 1: character "<" is the first character of a delimiter but occurred as data.
Code:
for(i = 0; i < links.length; i++) links.item(i).style.cursor = typeof type == '
Error 2: there is no attribute "onload".
Code:
var html = '<iframe onload="gXBL_setcursor();" src="http://gamercard.xbox.com/
Error 3: there is no attribute "frameBorder".
Code:
?g+'.card" scrolling="no" frameBorder="0" height="140" width="204">'+gamertag+
Error 4: document type does not allow element "iframe" here.
Code:
?meBorder="0" height="140" width="204">'+gamertag+'</iframe>';
Error 5: document type does not allow element "iframe" here.
Code:
?meBorder="0" height="300" width="150">'+gamertag+'</iframe>';
Error 6: XML Parsing Error: StartTag: invalid element name.
Code:
for(i = 0; i < links.length; i++) links.item(i).style.cursor = typeof type == '
I would imagine the popularity of this will shoot up tons once it validates simply because the sites that use SEO are going to be more inclined to use it and they are far more popular then most non-SEO sites.