A DOCTYPE Declaration is mandatory for most current markup languages and without one it is impossible to reliably validate a document. The DOCTYPE defines the version of the document like : HTML 4.0.1, XHTML 1.0, 1.1, ...
Please place a DOCTYPE declaration as the very first thing in an HTML document. For example, for a typical XHTML 1.0 document:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"> <head> <title>Title</title> </head> <body> ... body of document ... </body> </html>
For HTML 4.0.1, there are several one. Here is the most typical one:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
W3C spec: http://www.w3.org/TR/html4/struct/global.html#h-7.2
W3C faq: http://validator.w3.org/docs/help.html#faq-doctype
List of valid doctype: http://www.w3.org/QA/2002/04/valid-dtd-list.html
Choosing a doctype: http://htmlhelp.com/tools/validator/doctype.html