W3C & W3C Validation

The World Wide Web Consortium (W3C) is the main international standards organization for the World Wide Web (abbreviated WWW or W3). It was founded and is headed by Sir Tim Berners-Lee in October 1994.
Document Type Declaration
The DTD to which the DOCTYPE refers contains machine-readable grammar specifying the permitted and prohibited content for a document conforming to such a DTD. This is placed at the beginning of sources to determine which rendering mode to use:
e.g. For HTML 4.01

  1. HTML 4.01 Strict
  2. HTML 4.01 Transitional
  3. HTML 4.01 Frameset

HTML 4.01 Strict
Which does not have presentational elements like <font>, leaving formatting to Cascading Style Sheets and the span and div tags. SGML-based validator read the DTD in order to properly parse the document and to perform validation.
 The document type declaration for HTML 4.01 Strict is
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
HTML 4.01 Transitional
The Transitional DTD was intended to gradually phase in the changes made in the Strict DTD
The document type declaration for HTML 4.01 Transitional is
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
HTML 4.01 Frameset
This was intended for those documents which contained frames.
The document type declaration for HTML 4.01 Frameset is
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
W3C Validation and CSS Validation
The Markup Validation Service by the World Wide Web Consortium (W3C) allows Internet users to check HTML documents for conformance to HTML or XHTML standards. It also provides a quick method for web page authors to check their posted pages for mark-up errors.

http://validator.w3.org
http://jigsaw.w3.org/css-validator/