w3schools

 w3schools  is a site ostensibly built for web developers, but its resources often fall short of being definitive or following current standards.

Web users often use w3schools only because it is found at the top of search results, but other more definitive and standards-based sites and resources offer better learning tools. While W3Schools has structured tutorials and playgrounds that offer a decent learning experience, it would be a mistake to continue your education without learning from more reputable sources.

Better Resources
The following are are community driven efforts maintained by developers who continue to work in the field
 * W3C has created Web Platform Docs as the authoritative resource for documentation.
 * Mozilla Developer Network

Other good resources

 * Web Fundamentals (formerly HTML5rocks)
 * CSS-Tricks