• W3C HTML Validation Service - Validate your HTML/XHTML document against W3C Standard.
• Offline HTML Validator - Download and install this software to validate your HTML/XHTML pages offline.
• Starting with HTML + CSS - A W3C tutorial on HTML and CSS.
• XHTML1.0 - W3C XHTML 1.0. It offers the same flavors as HTML 4.0 and 4.01, reformulated in XML, with minor restrictions.
• XHTML1.0 - W3C XHTML 1.1. It is based on XHTML 1.0 Strict, but includes minor changes.
• HTML 4.01 Specification - A complete specification of HTML4.01, the most popular version of HTML.
• W3C Specifications List - A comprehensive list of W3C standards. You will find most latest versions available on this page.
• A Web standards checklist - Max Design provides a great resource for the standards world: A Web Standards Checklist. The list is meant to help folks understand the breadth of standards and provide a tool for developers.