xml:base support in old browsers

Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.

From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).

From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.

Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to WebExtensions if they can. See the "Compatibility Milestones" document for more.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.


Before using this code, check if your browser has native support for Node.baseURI.

These two functions aim to allow some support for xml:base.

Note however, that this code is not based on a thorough examination of the specs related to forming base URIs and could well have a number of errors. However, it is working in some of the cases tested.

var scheme = /(\w(\w|\d|\+|\- |\ .)*)\:\/\//;
function getXmlBaseLink (xlink, thisitem) {
	var xmlbase = '';
	if (!xlink.match(scheme)) { // Only check for XML Base if there is no protocol // tests for 'scheme' per http://www.ietf.org/rfc/rfc2396.txt'
		xmlbase = getXmlBase (thisitem); 
		if (!xmlbase.match(/\/$/) && !xlink.match(/\/$/)) { // If no preceding slash on XLink or trailing slash on xml:base, add one in between
			xmlbase += '/';
		else if (xmlbase.match(/\/$/) && xlink.match(/\/$/)) {
			xmlbase = xmlbase.substring(0, xmlbase.length-2); // Strip off last slash to join with XLink path with slash
//                        alert(xmlbase + '::' + xlink);

	var link = xmlbase + xlink;
	if (!link.match(scheme)) { // If there is no domain, we'll need to use the current domain
		var loc = window.location;
		if (link.indexOf('/') === 0 ) { // If link is an absolute URL, it should be from the host only
			link = loc.protocol + '//' + loc.host + link;
		else { // If link is relative, it should be from full path, minus the file
			var dirpath = loc.pathname.substring(0, loc.pathname.lastIndexOf('/')-1);
			if (link.lastIndexOf('/') !== link.length-1) {
				link += '/';
			link = loc.protocol + '//' + loc.host + dirpath + link;

	return link;
function getXmlBase (thisitem) {
	// Fix: Need to keep going up the chain if still a relative URL!!!!!
	// var ns = 'http://www.w3.org/XML/1998/namespace';
	var att, protocolPos;
	var xmlbase = '';
	var abs = false;
	// Avoid loop if node is not present
	if (!thisitem || !thisitem.nodeName) {
		return xmlbase;
	// Now check for the next matching local name up in the hierarchy (until the document root)
	while (thisitem.nodeName !== '#document' && thisitem.nodeName !== '#document-fragment')  {
		att = thisitem.getAttribute('xml:base'); // xml: namespaces MUST use 'xml' prefix
		if (att) {
			protocolPos = att.indexOf('//');
			var protocolMatch = att.match(scheme);
			if (protocolMatch) { // if has protocol, can stop
				if (abs) {
					var skipfile = (att.indexOf('///') === protocolPos) ? 3 : 2; // If the file protocol has an extra slashe, prepare to also skip it in the separator search
					var att2 = att.indexOf('/', protocolPos+skipfile); // Find first path separator ('/') after protocol
					if (att2 !== -1) {
						att = att.substring(0, att2 - 1); // Don't want any trailing slash, as the absolute path to be added already has one
				else if (!att.match(/\/$/)) { // If no trailing slash, add one, since it is being attached to a relative path
					att += '/';
				xmlbase = att + xmlbase; // If previous path was not absolute, resolve against the full URI here'
			else if (att.indexOf('/') === 0) { // if absolute (/), need to prepare for next time to strip out after slash
				xmlbase = att + xmlbase;
				abs = true; // Once the protocol is found on the next round, make sure any extra path is ignored
			else { // if relative, just add it
				xmlbase = att + xmlbase;
		thisitem = thisitem.parentNode;
	//return (xmlbase === '') ? null : xmlbase;
	return xmlbase;

Document Tags and Contributors

 Contributors to this page: bunnybooboo, wbamberg, Nickolay, Brettz9
 Last updated by: bunnybooboo,