I'm using an HTML5 "shiv" to

document.createElement("nav")

(along with the other HTML5 elements) so that Internet Explorer will recognize it, and I have a

<nav id="mainnav">

block in my document.

In Internet Explorer 7, selectors don't work right on these new elements. For instance:

jQuery("nav").width()

returns 960,

jQuery("#mainnav").width()

returns 960, but

jQuery("nav#mainnav").width()

returns null.

Works as intended in IE8 and WebKit.