MDN HTML
This page contains an overview of the basics of HTML. It states that HTML "defines the meaning and structure of web content." It provides a short list of some elements used in HTML and explains how most elements require opening and closing tags. Further down the page are links to an Introduction page (HTML Basics) which goes into more detail for beginners not accustomed to working with HTML; a Tutorials page (HTML Learning Area) which provides articles, code examples, and active tutorials; and a Reference page, which displays a list of every HTML element and attribute and how they all work.
I would use these pages as a way to brush up on the HTML knowledge I've already gained, as well as to learn about topics I have not yet covered. The list of elements is extremely helpful and a good tool to keep in mind in case I ever forget which element or attribute I need to use to make the page appear how I'd like. The addition of tutorials is also very helpful to be able to immediately practice HTML concepts without having to create a new document every time.
MDN CSS
This page contains a brief overview and history of what CSS is and what it is used for. It states "CSS describes how elements should be rendered on screen, on paper, in speech, or on other media." Like with the HTML page, there are pages for CSS Basics, CSS Learning Area, and CSS Reference which contains an alphabetical index of "CSS properties, pseudo-classes, pseudo-elements, data types, functional notations and at-rules." The Reference section also includes a list of key concepts, including syntax, units and values, box model, flexbox and grid, and several other major concepts. As someone who struggles keeping up with all the various CSS selectors, properties, classes, and such, this resource will be extremely handy to use as a reference point when I need to refresh my memory or get stuck trying to code something without success.
A trusted resource like MDN Web Docs is very important when looking for programming help, as one can be assured that the information they're accessing is vetted, accurate, up to date, and reliable. A website like this is a better resource than simply scouring the internet for other peoples' solutions for coding issues. While there is certainly good information to be learned from individuals as well, you can be confident using a resource like MDN Web Docs because they provide exhaustive lists of information and are kept up to date with the latest web standards.