The Mozilla Developer Network <https://developer.mozilla.org/en-US/docs/Web> is a really good resource if you’re doing any kind of web development. I’ve found all kinds of well-explained technical detail there that is simply missing from other sources (*cough* w3schools *cough*). Now Mozilla is trying to get some money from it. They’re not taking anything away from existing access, they are merely offering extra personalization features on a paid basis <https://www.theregister.com/2022/03/24/mozillas_mdn_subscription/>.