XML is extensible, and this is why. There isn't any good reason to encode markup as text content, only for the recipient to unencode it and then run its markup

XML is extensible, and this is why. There isn't any good reason to encode markup as text content, only for the recipient to unencode it and then run its markup parser a second time. If you want to use elements from the XHTML vocabulary inside an RSS document, the right thing is to just do it, like so: <rss:title>An <html:em>Emphasized</html:em> Title &amp; Stuff</rss:title> and when you need to cater to some widely-used aggregator that's too broken to handle this, the right thing is not to start breaking what you give everyone else, but to transcode this on the fly to whatever brain-damaged markup that particular aggregator version happened to be able to cope with (probably drop the namespace prefix and use CDATA, or failing that just strip all open/close tags and re-encode each &).