MDN HTML
HyperText Markup Language (HTML) is a markup language used for building the structure and content of web pages. In a sense, it's the most fundamental and basic layer of webpages. As a markup language, each element, or line of code, has two parts: an opening tag and closing tag. The only exceptions to this rule are elements that possess no content. MDN contains several deeper guides that go into deeper detail in certain aspects of HTML, such as comments and responsive images.
MDN CSS
Cascading Style Sheets (CSS) is a stylesheet language used for designing a web page in order to make it look good and appealing to users. CSS dictates how an element renders and looks. If a website looks bland and boring, users probably won't come back. Thus, CSS is just as important to web development as HTML or JavaScript is. MDN also contains several tools that help web developers write CSS, such as color mixers and border-radius generators.
MDN DOM
Document Object Model (DOM) is not a programming language like HTML, CSS, or JavaScript. Instead, it a web API that works with JavaScript to build websites. When a browser loads a web page through an HTML document, a DOM tree is built. A DOM tree allows JavaScript to access elements and begin to implement functionality into a web page. Without a DOM tree, JavaScript can't do it's job properly. Though, DOM is not a part of the JavaScript language, it simply allows JavaScript to do it's thing.
MDN JavaScript
JavaScript (JS) is a scripting language responsible for web pages to be complex and more functionality beyond accessing other web pages and displaying text on a screen. JavaScript gives programmatic control over elements in a web page, allowing for great manipulation. JavaScript works with the DOM in order to do this. Some JavaScript features include being able to download files from websites, have popup menus, and complex animations. Despite it's name, JavaScript has no relation to another programming language, Java.
Summary
HTML builds a website's structure and content, CSS builds a website's look and feel, JavaScript and DOM work together to build a website's functionality. It's impossible to know everything, meaning eventually, you will have questions that need to be answered. Sooner or later, you will encounter a problem too complex to solve on your own. That is when a trusted resource is needed. A trusted resource is a reliable source of information, this can come in the form of a website, book, or a person that you know you can trust for further information on a certain topic. The Mozilla Developer Network can be considered a trusted resource when it comes to web development, specifically HTML, CSS, JavaScript, and DOM. Whenever developing a website, if you have any questions, you can trust MDN to give you a proper answer.