The MDN HTML page gives an introduction to HTML. The page links to reference pages for each element and attribute.
The MDN CSS page introduces CSS. The page links to reference pages for each property, selector, combinator, pseudo-class, psuedo-element, at-rule, and function.
The MDN DOM page discusses what the DOM is and how it is used.
The MDN JavaScript page introduces JavaScript. The page links to reference pages for each built-in object, expression, operator, statement, function, class, and error.
The MDN HTML, CSS, DOM, and JavaScript reference pages gives a basic introduction about each topic and then discusses specifics, such as elements, properties, functions, etc. I would use the MDN pages if I needed help while coding. Trusted resources are important when looking for coding help because your code is not going to work correctly if you get wrong and/or out-of-date information.