The jQuery documention for the attr method states that:

Attribute values are strings with the exception of a few attributes
such as value and tabindex.

And that does seem to be the case. Consider the following element:

<input type=text id=example tabindex=3>

The following line does indeed show number, not string:

alert(typeof $(#example).attr(tabindex)); //Number

Now, the thing that's confusing me is that when using the DOM method getAttribute, you get a different result:

alert(typeof $(#example)[0].getAttribute(tabindex)); //String

Looking at the jQuery source for the attr method, it appears that jQuery simply returns what getAttribute returns, so why is there a difference? Here's the relevant lines of the jQuery source:

ret = elem.getAttribute( name );

// Non-existent attributes return null, we normalize to undefined

return ret === null ?

         undefined :

         ret;

And here's a fiddle to demonstrate the issue. Just to confuse matters further, I've tried it in Chrome 15, Firefox 8, IE8 and IE7, and all behave as described above, except for IE7, which alerts number for both (which is what I would expect to happen).