A DOCTYPE Declaration is mandatory for most current markup languages and without one it is impossible to reliably validate a document.

The W3C QA Activity maintains a List of recommended Doctypes that you can choose from, and the WDG maintains a document on "Choosing a DOCTYPE".

No Character Encoding Found! - An HTML document should be served along with its character encoding.

The line in the HTTP header typically looks like this:
Content-Type: text/html; charset=iso-8859-1

eh! eh!