How strange! IE does, however, understand it when you use innerHTML instead of DOM methods.
function escapeHTML(s) {
return s.split('&').join('&').split('<').join('<').split('"').join('"');
}
function qualifyURL(url) {
var el= document.createElement('div');
el.innerHTML= '<a href="'+escapeHTML(url)+'">x</a>';
return el.firstChild.href;
}
A bit ugly, but more concise than Doing It Yourself.