Documentation Sources
Summary
HTML Resource
The HTML documentation resource provided by the MDN provides an overview of the key terms associated with HTML. It explains the basic concept of hyperlinking (or linking to other pages). It explains the basic syntax of HTML and provides some examples of certain HTML "tags" that can be used to create a web document. Not only does it provide an introduction to HTML, but also tutorials for intermediate and advanced programmers alike.
CSS Resource
The CSS documentation resource linked above provides a comprehensive overview of the language used to style web documents. It explains the concept of specificity (or how certain styling rules can be overridden). It explains the difference between user-agent style sheets (those provided by the web browser) and author stylesheets (or custom ones created by web developers). It also provides examples of CSS syntax and practice exercises.
DOM Resource
The DOM documentation resource linked above explains that the basic functionally of the DOM is merely to store a representation of the document structure in the memory of the web browser. Unlike the other web standards that are created by the W3C, the DOM is merely a memory interface for the webpage. In order to manipulate this memory interface, another programming language is required -- which is where JavaScript usually comes into play.
JavaScript Resource
Unlike the markup languages listed above, JavaScript provides a way to directly program the website. JavaScript is a programming language, while HTML and CSS are markup languages. The MDN documentation explains that it is not only the de-facto scripting language used in web browsers, but also in Node.JS (a native runtime for JavaScript used often in web servers) and CouchDB (a database system that uses JSON requests rather than SQL statements). Like the HTML and CSS pages, JavaScript documentation provides plenty of practice exercises and in-browser test snippets.