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MDN Web Docs: A resource for developers, maintained by the community of developers and technical writers and hosting many documents on a wide variety of subjects, such as: HTML, CSS, HTTP, JavaScript, Web APIs, Web components, Graphics, Audio, video and multimedia, MathML.
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StackOverflow: A massive resource of questions and answers having to do with coding. If you have a question regarding web development or coding in general, chances are it has already been answered on StackOverflow.