MDN HTML

The HTML reference page first defines what HTML is. Explaining that it is the most basic building block of the web. It lists some key resources such as an introduction to HTML, a link to their learning area containing many tutorials, and a reference section containing details about every element and attribute. Beyond that they have beginners tutorials on this page as well as advanced topics such as CORS enable images and attributes. The last part is links to reference pages showing the elements attributes, global attributes, and guides to media types and formats. I would use this page and have in the past to look up explanations of elements and attributes to make sure I've not missed anything or to try and fully understand what it is I am doing. This all helps understand the structure and markup of web content.

MDN CSS

The CSS page is generally the same as the HTML one. It supplies a definition of Cascading Style Sheets. CSS describes how elements should be rendered on screen, on paper, in speech, or on other media. They have some key resources listed such as an introduction to basic CSS, CSS tutorials, and links to reference pages. I have a feeling at some point I will end up on their tutorial page on using CSS to solve common problems. There's a link to CSS layout cookbook which brings together common layout patterns, I've landed on that page a few times to review layouts. In summary this page brings together all the information you would need to work with CSS.

Summary

Trusted resources I think are crucial as they supply up-to-date and accurate information. If you're reading inaccurate information that will likely lead to bugs or non-compliance with web standards. This could make you website inaccessible. Mozilla is trusted and has a good reputation so it its likely to be cited and recommended by industry professionals. Using untrustworthy information can lead to security issues as well. So relying on a trusted resource is like constructing a building on a firm foundation versus building it atop an unstable base this determines the soundness of the structure.