HTML

The HTML page on MDN provides a definition of HTML, an introduction / basics section, a tutorial and example section, as well as a reference guide. I would use these resources, especially the reference guide, as it contains lists of all elements and attributes used in HTML, how they work, and how to use them.

Mozilla Developer Network: HTML

CSS

The CSS page gives some history on CSS, as well as giving the user access to an introduction to using CSS, some tutorials, as well as a comprehensive reference for all of the properties and corresponding attributes in CSS. I've actually used the guide on the main assignment a bit to jog my memory of how to use the box-shadow property, and I will continue to use the reference guide to provide clarity on any confusing attributes. It's really handy to know which properties can go with other properties, such as flex or grid specific properties with flexbox and grid layout.

Mozilla Developer Network: CSS

DOM

The DOM page describes what the Document Object Model is and the basics of how it works, as it's the model loaded into the browser when a page is inspected to then be modified with. There is also more information on the DOM such as standards the DOM uses, and an external Wikipedia page covering the DOM.

Mozilla Developer Network: DOM

JavaScript

The JavaScript page on MDN is sectioned differently compared to the others, as there is a guide and reference provided at the top of the page, but then the page starts going into tutorials ranging from basic to advanced, where different users can browse different aspects of JavaScript. If I had any questions or needed a refresher for something in JavaScript, I would use this page to find what it was I needed help with.

Mozilla Developer Network: JavaScript

Conclusion

A trusted source, such as the Mozilla Developer Network web docs, is a source that users can use reliably and safely for themselves. The user shouldn't need to worry about a copy-and-pasted code segment browsing their computer files or digging through passwords when they're trying to find a solution to a programming problem. For the previously listed reasons and plenty more, it is important to identify that a source is trustworthy before using code provided by the source.