Why no script implements
Adding the implements attribute to scripts would allow user agents to recognize the purpose of a script, and not bother to download scripts it already had in cache, or for which it provided a native implementation.
Toby Inkster provided a pretty good argument for including it in http://lists.w3.org/Archives/Public/public-html/2009Sep/0538.html
Simon Pieters provided the more subtle counterargument in http://lists.w3.org/Archives/Public/public-html/2009Sep/0540.html
Basically, the warning about how imprecise and unreliable DOM feature testing can be ( http://dev.w3.org/html5/spec/Overview.html#dom-feature-strings ) would also apply to @implements.