- MDN Web Docs — Owned by Mozilla. A great resource for basically all the topics in this list. This is probably the single most useful site on this list.
- Can I use — Tool for determining whether a specific browser supports specific HTML/CSS/JavaScript features
- CodePen — Useful site for writing HTML/CSS/JavaScript directly in the browser and seeing it rendered.
- HTML, CSS validators
- "wtf is a validator?" — For compiled languages like Java, C/++, etc. you don't need one of these because your code gets validated when you build. HTML/CSS aren't programming languages, they are markup/stylesheet languages which don't have a build step so they will attempt to run even if they aren't "valid". Many (most?) professors will make you scan your HTML/CSS with a validator to make sure it's correct.
- Chrome DevTools — DevTools is a feature that comes built into your Chrome browser. Getting the hang of using it can save you a looooot of time when debugging issues. I'd personally try to find a YouTube video that gives an overview, like maybe this one (or just search around until you find another that fits your fancy). Being able to open and navigate DevTools is a super useful skillset.
- JavaScript @ MDN — JavaScript homepage at MDN. Mostly useful for looking up language syntax.
- Introduction to Programming with JavaScript @ Launch School — Free online book, one of the best resources for learning the main features of JavaScript. Some other schools have even adopted this into their own curriculum. I'm also biased because I helped edit the book.
- Ajax guide @ MDN — A short intro on Ajax and links to relevant tools. Ajax isn't actually one specific tool, but an approach for using a few different tools together in a certain way.
- tl;dr — In the old days of the web anytime you clicked something it would basically refresh the entire page. Over time developers have shifted to where when you interact with the page the browser will make requests in the background and update content without requiring a full page reload. There are still plenty of pages that behave the original way but, generally, the more interactive pages on the web are powered by Ajax (which is really just a specific way of using JavaScript)
- XML introduction @ MDN
- XML is a bit less common in the wild anymore, it's much more common to use something like JSON instead, but you'll almost always have to learn XML in school.
- Unless your school specifies something else I'd use either VS Code or Sublime Text. The former is probably the most popular editor in the industry right now, the latter is suuuuuuper well optmized but a little older.