While the proliferation of the Web and HTML has driven a revolution in business and communication over the last fifteen years, the flexibility of HTML is ironically one reason the revolution has not been as widespread as it could still be.
HTML as a language is syntactically very forgiving. Most browsers spend extra processing time dealing with poorly-formed HTML pages. Quirks mode is a state most browsers enter when encountering a page with malformed HTML. While essential for backwards compatibility with older web pages, it is inefficient and often renders poorly to the screen.
XHTML is a language that provides the mark-up flexibility of HTML, but it also conforms to the stricter syntax of XML. This allows both browsers and more importantly, web server software, to render items more efficiently and also makes it easier to allow different types of media to be viewable within a browser. The XML format is an industry standard and thus data and media from other XML-based languages such as MathML can be easily incorporated into larger XHTML documents.
Since XHTML is at its core a stricter version of HTML, most web developers will have little problem moving to XHTML. Essentially it means working more carefully and paying attention to how the tags are written – and most importantly, closed. XML is a case-sensitive language, and therefore so is XHTML. Thankfully most quality development environments contain features to enforce the additional syntax rules. Simply put: every web developer should be using XHTML.