The W3C is the source for all things Web-related. Following are two areas of particular interest.
Free tutorials on Web languages, technologies, and development are available at http://www.W3Schools.com/.
The W3C Web Accessibility Initiative at http://www.w3.org/WAI/ provides a great deal of practical information about designing applications and interfaces for the broadest range of users. Allaire supports user accessibility in its product interfaces and is striving to fully implement the WAI recommendations.
Simple steps, such as providing a text alternative for images, graphics, and animations, and using clear descriptive text for page elements and navigation, can make a world of difference to a visually impaired user's experience of your site. Validation tools are available on the site to check compliance with established standards.