I remember just using PHP sessions back then on a XHTML document produced parse errors. Because PHP added the session to the query strings of links and used the raw & character instead of & for separating params in the query string. Thus causing a XML parse error.
There was a push to prevent browsers to be too lenient with the syntax in order to avoid the problem that sloppy HTML produced (inconsistent rendering across browsers)
There was a push to prevent browsers to be too lenient with the syntax in order to avoid the problem that sloppy HTML produced (inconsistent rendering across browsers)