The HTML section of this page provides information for beginners as well as experts on HTML. There are links on the homepage for HTML that take readers to specific pages for each of the most common HTML elements. For beginners, they provide an HTML Basics article to teach about what HTML is and how to use it. They also offer tutorials for how to solve different problems using HTML. Finally, they have an "extensive" HTML reference section that provides information about each element and attribute in HTML. I would use this resource if I am trying to create something with HTML and struggling or if I want to expand my knowledge and use more of the elements in HTML.
Similarly to the HTML page, the MDN reference page for CSS provides information for beginners and seasoned developers about CSS. New developers can use the CSS basics article to learn all about CSS and how to get started using it. They also have tutorials on their page to teach how to do new things with CSS. Finally, they have an exhaustive list of CSS properties and concepts in their CSS reference guide. I would use this (and have in the past) to learn how to acheive a specific style with CSS and to learn more about different things I can do with it.
It is important to have a trusted resource to visit when looking for programming help for several reasons. First, MDN's comprehensive guides provide information and help on just about every thing you could create with HTML, CSS, and more. It is helpful to have one place to go to that will provide answers for all of your questions. Additionally, using a reputable source will ensure that you will be creating valid and safe code. If you use a source with a weaker reputation, you risk creating code that could help hackers access and control your site, or at least you may end up writing invalid code.