MDN HTML
The HTML documentation at MDN provides a ton of useful information. You can find everything you need to know about elements, attributes, and how to use it all. Since HTML is the bones of the page you need to understand semantic markup and accessibility requirements.
MDN CSS
MDN provides detailed explanations of selectors, layout techniques and anything else you want to know. There's a ton of examples on how properties work. I've used this for button research and layout design. Typically I'll have a window open just sitting on MDN for when I can't remember something or want to try something new.
MDN DOM
This page on MDN explains how Javascript interacts with HTML and CSS. The information here would be used when you need to respond to user actions or change page content using Javascript.
MDN JavaScript
Reference and learning material for Javascript is provided within these pages allowing you to add functionality to your websites. They cover everything from basic to advanced concepts.
Summary
MDN is a community-driven resource for developers. They make sure their information is accurate and up to date. The site is maintained by Mozilla and the community. It is important to use trusted sources such as MDN since they give you standardized information that is based off the current industry standards.