MDN HTML
This page holds a wealth of information related to HyperText Markup Language (HTML). Here you can look at the basics of HTML and go through tutorials on how to do various things in HTML such as forms, tables, and media embedding. This page also holds links to references that contain all sorts of useful information for anyone trying to write HTML. Because this site has so many resources pertaining to HTML this would be a good place for anyone to learn or recall HTML.
MDN CSS
This Page has information pertaining to Cascading Style Sheets (CSS). This page contains tutorials, resources, and background information on CSS. Something I found particularly helpful was this page has resources containing code for some standard CSS template formats. This site is very helpful for finding specific elements and functions in CSS.
MDN DOM
This page contains information on the Document Object Model (DOM). This page contains background information, current attributes, and previous attributes for DOM. While bare, this page provides a good overview on what the DOM is and its job when you code.
MDN JavaScript
This page pertains to JavaScript. The page contains tutorials, tools, and information for JavaScript. Sites like this are incredible useful when looking for built in functions, this always helps from writing unnecessary functions.
Summary of Documentation
Trusted resources like those covered above are incredibly important when working on any code. Making sure you use trusted resources usually ensures the most efficient and standard makes it into your code. It also ensures that you have access to the most up to date methods for completing certain tasks.