W3Schools.com

W3Schools is an online educational website for Front end developers, it provides resources, courses and information for basic web development aspects, such as HTML, CSS, JavaScript (And it's most popular library, jQuery), but also back end programming such as SQL, PHP.

Although at first glance, the website only provides support for web development, it also provides many other programming languages.

W3Schools also provided a full fledged ready-to-use CSS framework called W3.CSS, and two JavaScript library/framework called AppML and W3.JS.

Supported languages
W3Schools, is aimed for full stack development, so it would provide support to:
 * HTML.
 * CSS.
 * SASs.
 * JavaScript.
 * jQuery.
 * React.
 * AngularJS.
 * AJAX.
 * SQL.
 * PHP.
 * ASP.
 * Node.js.
 * Raspbery Pi.
 * Git.

Although originally made for web development, it also provides support for other programming languages over time, these include but not limited to:
 * Python.
 * Java.
 * C++.
 * C#.
 * R.
 * Kotlin.
 * Go.

Compiler and Editor
One of W3Schools' pride is their code editor and compiler, which works by using server side processing and outputting the results to the end user.