...Every HTML document you create requires a document-type declaration--DOCTYPE. This declaration begins the HTML document and tells a validator which version of HTML to use when checking its syntax.

...The intention behind document switching is to ditch old practices and write standards-compliant code. By switching to the strict mode you can practice writing code that works the same way on all modern browsers. This goes for Internet-enabled devices many people simply overlook—PDAs, cell phones and other wireless accessories with limited rendering capabilities. Pages authored in the Standards mode will work much better than the Quirky ones. The Standards mode "excludes the presentation attributes and elements that W3C expects to phase out as support for style sheets matures. Authors should use the Strict DTD when possible, but may use the Transitional DTD when support for presentation attributes and elements is required.

Since you will in effect be completely reworking your website to include the DOCTYPE, I would recommend 4.01 strict. Below is a link for the different types. Or if 4.01 strict proves to be "outta sight" then XHTML 1.0 transitional.

http://www.w3.org/QA/2002/04/valid-dtd-list.html

Further reading which may be "simpler" than what you will find at W3C is below.

http://www.aspnetresources.com/articles/doctype.aspx


Now with all of that out of the way, if you are using CSS inline (or CSS with each page), by converting to an external CSS all of your pages will load much faster along with including the DOCTYPE too.

Need additional help? AS! has a tutorial on using FireFox with Tidy inline you may find beneficial.

You could just cut and paste the DOCTYPE for HTML 4.01 strict into each page. Then do nothing more. But some of the style and presentation code you use would or could not work. So that is why going though the validation process to "purify the HTML coding" really means that all of your effort gains the greatest benefit. To You and everyone who will visit.

Although there is really not much to be gained from using XHTML at present, many people have started using it. Then it becomes relevant that validation means different things for XHTML. The reason is that the metalanguage, XML, is considerably less powerful than SGML.

For further reading:
XML The Extensible Markup Language is a profile or restricted subset of SGML as a metalanguage. XML is used to define application-specific XML vocabularies, or XML markup languages.

SGML The Standard Generalized Markup Language (ISO 8879:1986) is historically the dominant (meta-)markup language.