MDN HTML
The MDN HTML reference page offers a brief overview of what HTML is and does. Beyond that the page offers tutorials from beginner level to advanced.
MDN CSS
Like the HTML reference page the CSS page gives an over view of what CSS is. There are tutorials from basics to advanced. Along with the tutorials there's a reference section, a guide section, and something called the "Layout cookbook" which I'm going to check out in more depth when I have some time.
Summary
In summary reliable trustworthy sources of instruction and reference are important for developing good code. MDN has been providing just that since 2005. Whether you just want to use MDN as a reference source, or you want to use it as a learning hub it's got it all. Want a step by step tutorial on how to become a front-end web developer it's there in the learn section. I've bookmarked it for my own future and present use. In fact I've been using MDN without even knowing it. VSCode uses MDN for inline reference material.