Other than layout and rendering, implementing HTML, ECMAScript and CSS is genuinely easy. There’s a lot of it so that it’ll take you a long time, but it’s very

Other than layout and rendering, implementing HTML, ECMAScript and CSS is genuinely easy. There’s a lot of it so that it’ll take you a long time, but it’s very much not hard, because the HTML and ECMAScript specs fully spell out the algorithm, telling you exactly what you must do (or, more precisely, what you must be equivalent to doing: e.g. “implementations must act as if they used the following state machine to tokenize HTML”), so it’s largely mechanical. This is very unusual in specs. I wish it were less so. Take a look through https://html.spec.whatwg.org/multipage/parsing.html https://html.spec.whatwg.org/multipage/parsing.html. It’s verbose but very approachable, very implementable.