As we’ve seen in other articles in this series, XHTML offers a lot of advantages over HTML. It’s stricter, cleaner, more robust, and extensible. However, some modern browsers such as Internet Explorer don’t understand XHTML; they only understand HTML 4. Fortunately, the two languages are very similar and, by bearing a few guidelines in mind, you can write valid XHTML documents that can be processed correctly by HTML 4 browsers.
Read all about XHTML – the successor to HTML – and learn how to build XHTML-compliant Web pages.
In this article you take a look at the concept of document type definitions, or DTDs, and explore the three DTDs available to you when writing XHTML documents.
XHTML, first introduced in 2000, is billed as the successor to HTML. It’s short for Extensible Hypertext Markup Language. XHTML 1.0 is essentially a reworking of HTML 4 in XML – Extensible Markup Language. As such, HTML 4 and XHTML 1.0 are very similar.
XHTML is stricter than regular HTML, as you’ll see in a moment. While this extra strictness requires a bit more effort when creating XHTML pages, it does mean that those pages are very easy for computers to read. HTML, in contrast, is notoriously difficult for browsers to interpret — which is partly why no two browsers seem to display a Web page in the same way!