MDN HTML
This page contains links to an introduction to HTML itself as well as basic structures like tables and forms. Learning any coding language always requires syntax knowledge, and knowing about elements you're basically guaranteed to use is the first step in expanding your capabilities. In addition, you can find answers to common problems, which may be an easier first step in the research process. As you become more advanced, there are topics you can explore as well as list of all HTML elements and attributes. Honestly, I prefer using Google to find answers rather than scrolling through documentation websites like this one because I feel like it's easier to narrow my focus, but since there are lists of HTML elements and attributes, I may use this site to at least explore options.
MDN CSS
Being a styling language rather than just building blocks, there's a lot more links on this page, but I'll try to summarize the best parts. As with HTML, beginner tutorials and a list of every single CSS concept can be found. Because of the sheer amount of CSS concepts, this may be a little less helpful than the HTML one versus looking up the solution to a problem directly, but it does exist. The references section calls out some pretty interesting topics ranging from general to specific. One of the general ones is shorthand. I would personally never use it because I'd rather type a little more than remember two different syntaxes for one line of code. Plus, the extra code makes the line make more sense when you come back to it later. Overall, I probably wouldn't use this page much for the same reason as HTML, but it's definitely more interesting.
Summary
An extreme reason for not using an untrusted resource would be copying over malicious code hidden in the answer. I don't know how possible this actually is, but you never know. Other than that, trusted resources are better for beginners. If you don't know what you're doing and copy over some code that doesn't work, you may not know why. You may even go as far as assuming your theoretical concept doesn't work even if that block just wasn't made for your situation. Also, you may get to an answer you're searching for faster. This kind of ties back to the previous point, but it has to do with working code as well as knowing your situation.