For IE 5 and 6, you need to use a different AJAX request, using ActiveX.
If so, is it good practice to try to incorporate these older browsers, or do most people not bother?
3:17 am on Jul 3, 2012 (gmt 0)
Correct. See also this thread: [webmasterworld.com...] IE5 and IE6 are both basically non existent, so you shouldn't even bother with those (unless you have a specific requirement to support IE6 for some corporate application where IE6 is still used). As explained in that other thread, supporting old versions of IE is actually quite easy, but really, they are dead browsers.