Hobbies Educational websites for programming

[Freedom]

This thread is for posting resources/websites/books/links/etc that are aimed for educational purposes in regards to learning to program, the only condition is that it must be free. You can also post your experience if you have used any of them.

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I'll start by mentioning the infamous W3Schools, which I've used superficially to look at HTML and CSS, however I've read on many occasions that the website is not that good or exhaustive, and that their "certificates" are pretty much a joke; it is however useful for a quick search that you need as a reminder on something.
Take what I've said as purely anecdotal, as I'm not an expert on this.

The next one is MDN Web Docs, which I believe is somewhat exhaustive (again, only looked at HTML), but it's just pages of information for you to read, unlike an interactive guide that will teach you step by step.

Next is Codecademy which I thought was good at first, but it was kind of a disappointment when I did the free course of Regular Expressions, because it was so basic.

Finally, I've heard about The Odin Project with positive reviews, but never used it.

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So, have you used any (mentioned here or not), and would you recommend them?
And is there one you'd recommend for a particular language/other?

 

[Cavalier Cipolla]

CPU unofficial opcodes

Unofficial opcodes, sometimes called illegal opcodes or undocumented opcodes, are CPU instructions that were officially left unused by the original design. The 6502 family datasheet from MOS Technology does not specify or document their function, but they actually do perform various operations.

www.nesdev.org

nesdev wiki has some useful resources for NES programming. Here's one such page.

 

[Crimson Fucker]