title: JQuery , set attribute for <a> tag
body: <p>i am use $(expr).attr("hash",value) to set a "hash" attribute for HTML anchor element "a" 
  but the Jquery would not do that. but if i change expr to "div", then i can set
  "hash" attribute to "div" tag.</p>
  
  <p>is this behavior a xhtml specification? i can set "id" of "a" tag attribute. since id is a built in attribute for html "a" tag. </p>