There isn't a difference in readability, XHTML is just stricter. Much of the HTML that exists is invalid HTML/XHTML, and the XML parser used for XHTML would simply error out. Most XHTML pages were served as HTML, due to mimetypes being wrongly configured, so no-one ever noticed.

The XML parser was supposed to be faster, and allow any XML to be embedded in the XHTML (SVG, MathML, etc). This stuff was designed to change the shape of the web (especially since mobile phones weren't very powerful in those days).