From http://www.w3schools.com (Copyright Refsnes Data)
Writing your pages to the web standards will improve the quality of your site.
XHTML is the latest HTML standard. XHTML is a reformulation of HTML 4.01 in XML.
Writing your pages to the latest HTML 4.01 standard will bring you as close as possible to the latest XHTML standard.
Using Cascading Style Sheets (CSS) is the preferred way of separating content from the style in quality web pages. By using CSS you can store all the information about the style of your web pages in one single document.
All modern web browsers have support for both the CSS 1 and CSS 2 standards.
Using CSS will improve the quality of your web site and increases the readability for many different browsers. It will also greatly reduce your web site development costs.
A web validator is a software program that can check your web pages against the web standards.
When you use a validator to check an HTML, XHTML or CSS document, the validator will return a list of any errors found, according to your chosen standard. Normally the validator also returns the line numbers where errors are found.
Make sure you make it a habit to validate all your web pages before publishing.
Read more about page validation.
WAI stands for the "Web Accessibility Initiative", and is initiated by W3C.
WAI coordinates with organizations around the world to increase the accessibility of the Internet through six primary areas of work: technology, guidelines, tools, education, research, and development.
You can improve the quality of your web site, and make your information available to more people (and browsers) by writing your pages according to the WAI guidelines.
You will learn more about WAI in a later chapter of this tutorial.
From http://www.w3schools.com (Copyright Refsnes Data)