If you have such an unrestrictedly typed code (old code or third party code), that is used variables without declaring. One variable declared within some function/scope and used from somewhere else(it will be undeclared there) and you can't rewrite/change them, then you should not go for "use strict;" mode because it will break the code.
HTTP request method, URI, version HTTP headers: Content type, User agent, Host, Accept languages HTTP body
data-* is used to store data private to the page or application in html element no ajax call or server side queries
elegant forms user interface enhancements reduced need for JavaScript and forms validation native to HTML Works well across browsers and platforms