Useful Resources

HTML

• HTML Tag Reference
  via w3schools.com
• W3C Markup Validator Service
  Check the HTML markup of web documents.
• HTML5 Boilerplate
  The web's most popular frontend template

CSS

• CSS Reference
  via w3schools.com
• Learn Layout
  Site that teaches the CSS fundamentals used in any website's layout.
• W3C CSS Validator Service
  Check Cascading Style Sheets.
• CSS Lint
  A static analysis tool to assess the quality of CSS code.
• Media Queries
  Some good examples of sites built using principles of responsive web design.

C#

• C# for absolute beginners

SQL Server

• Database Fundamentals
  

JavaScript

• Superhero.js
  A collection of great articles, videos, and presentations on creating, testing, and maintaining a large JavaScript code base.
• JavaScript Reference
  Fairly exhaustive reference for JavaScript; via the Mozilla Development Network.
• JS Lint
  A static analysis tool to assess the quality of JavaScript code.

jQuery

• Learn jQuery
  jQuery learning center with a complete guide to the framework.
• jQuery API Refernece
  Complete documentation on the jQuery framework API

Ajax and Web Services

• Ajax Tutorial
  via w3schools.com
• Web Services Tutorial
  also via w3schools.com
• Official JSON Site
  Documentation and language-specific libraries for JavaScript Object Notation.

Tools

• Can I Use?
  Compatibility tables for support of HTML5, CSS3, and more in desktop and mobile browsers.
• Code Pen
  Web-based programming environment to experiment with, learn, and teach HTML, CSS, and JavaScript.
• Code School
  Web-based interactive courses on many popular web development programming languages and technologies.
• Web Page Test
  Web-based tool to measure the performance of any web page.

Standards

• RFC 2616: HTTP/1.1 Specification