It's great to use DocBook for writing books! (Actually, I don't think this is true for most books, but you do you.) But I would argue that processors that are designed to take HTML as input are not at fault for failing to process non-HTML documents. (Many, but not all) web browsers can render and style XML like this, but I'd argue that this is -- somewhat ironically given the article's subject -- probably not the best approach to putting DocBook files on the web.
The only way to build a fully conformant browser is to support rendering HTML as SGML with a DTD. Toy/specialty renderers skip over that step since you can assume 99.9999% of web documents will be HTML. However, they would completely fall apart on standard SGML, styled XML, etc.