You could probably also do something similar to the article, providing your document is XML-valid, using XSLT. This would keep the advantage that you just load your document in a browser (having added a temporary stylesheet link either manually or dynamically) but would support more powerful validation. It could be made into a browser extension so you're just a one-click away from validation with in-page feedback.
Of course, you can also validate XHTML using the provided DTDs if you take out the SGML-specific bits, using a (DTD-)validating XML parser (or indeed use XSLT as you say). But SGML has tag omission/inference, empty elements/HTML "void" elements, attribute short forms, unquoted attributes, Wiki syntaxes/markdown, and other features desired or even required for mere parsing mainstream HTML. As to using XHTML, I'm not sure it has a bright future. Last I've worked with it was like ten years ago (xsl-stylesheet processing instruction etc. on IE6; newer browsers don't seem to support it anymore).
FYI: SGML is way more powerful than XML, and its grammar formalism is identical to that of XML because XML was designed as a simplified subset of SGML. Though Relax NG, and also XML Schema, can theoretically express richer grammars than SGML, they can't be used for HTML due to lack of tag inference and other short syntax features, and the features that Relax NG has over SGML benefit data-oriented rather than document-oriented use cases (eg. the use cases that JSON and other serialization formats are arguably better suited for than markup in the first place IMO).
sgmljs.net can run (also) in the browser, so can also validate HTML5 in the browser.
I've had success with xhtml and xslt processing on the server-side. The fact that it's xhtml is incidental; that it's xml-valid means I can use a wealth of established tools to modify the content, and easily write my own. I don't mind too much about xml support in the browser, although I've found xslt to still be supported in chrome - e.g. for rendering RSS feeds as HTML.
