You can retrieve values of custom (or your own) attributes using getAttribute
. Following your example with
<div id="geoff" data-geoff="geoff de geoff">
I can get the value of data-geoff
using
var geoff = document.getElementById("geoff");
alert(geoff.getAttribute("data-geoff"));
See MSDN. And although it is mentioned there that you need IE7 to get this to work, I tested this a while ago with IE6 and it functioned correctly (even in quirks mode).
But this has nothing to do with HTML5-specific attributes, of course.