// Need to wait until doc has loaded.
onload = function(){
var ids = [];
var elms = document.getElementsByTagName("*");
// Loop stops when elms[k] returns null
for(var k=-1, elm; elm=elms[++k]; )
if(elm.id) // if id exists, and is not zero length
ids.push(elm.id);
// Now ids array contains all id strings
}