I knew it ! ;))

At the time HTML was the recommended spec, very few web designers bothered with a <!DOCTYPE>, or with valid code at all, so in practice an HTML document could be any text file containing any combination of text and HTML tags.
These days, the needs of accessibility, search engine optimization, document consistency for JavaScript manipulation, and support for international characters all combine to require more of our HTML. The minimal HTML document has gotten a lot bigger.
Every XHTML document should start with a <!DOCTYPE> that tells the browser what version of XHTML the document is written in. In practical terms, this tells browsers like Internet Explorer and Firefox to use their most standards-compliant (and therefore cross-browser-compatible) rendering mode. The exact form of the <!DOCTYPE> depends on whether your document is HTML (fine for most purposes) or XHTML  (enables the page to be processed as XML):