Error raro

[robert66]

Gracias,me dice

Fatal Error
Could not validate this document, because it does not include a DOCTYPE Declaration. A DOCTYPE Declaration is mandatory for most current markup languages and without such a declaration it is impossible to validate this document.
Place a DOCTYPE declaration in your document and retry to validate. For third-party documents, you can choose a DOCTYPE from our DOCTYPE-bar.
Here you can find a list of valid [X]HTML-DOCTYPES, maintained by the W3C.

pero desconozco como se hace esto.