In case you want to protect your HTML5 / JS code, what are some preferred, standard, Itch.io compliant ways to do so?
Protect from what? Being used as intended by legitimate users? Because that is what you will get. As is what all the other DRM implementors created, when they did it to their works. DRM has been around long enough to give plenty of examples, how legit users now can no longer access their bought product. As a concept if fails on all aspects, as it does not even protect against piracy or plagiarism.
Protection against law abiding and aware of the law users is implicit. You do not need to register a copyright to have copyright. But you can spell it out, if you put your software under specific licenses, if applicable.
Protection against non law abiding people will only (!) hurt the law abiding users. If your stuff is worth anything, the criminals will just steal it anyways, no matter what you do. And they will re-release it without the precious drm. And if your stuff ain't worth stealing, be happy, if people share it, you would not have sold it to them anyways.
Also, how do you buy a html5 game? If I give you money I only have access to all the downloads, even if you set a minimum price for files later. But if you do not have any downloadable files, all this is is a non issue. The downloads you see, are people playing it in the itch app, as it caches the games and that counts as a download somehow.
You can do drm, if you need to, there are no rules on itch against it. But also no support. And from my perspective, not even AAA and AA and A companies do drm in any good or trustworthy way. There are drm systems that, if on a game, will not be bought by some people, because it is just garbage - and not because they cannot pirate the game. Search for "denuvo problems" on the net.
So what makes you think, you, a single amateur developer can implement a drm system that is any good and will do what you want it to do, without harming or bothering and hurting your legit users?
Sorry for ranting here, but there is a thread like this every other week by a developer that has not even anything to show (yet) that looks like it even needs protection. And you probably did not even want to actually implement drm, so this is not specifically meant for you, but for others searching the forum and finding this.
Wanting a little peace of mind after releasing hard work is understandable, as there is always the danger that someone just copies it and claims he made the stuff. Small indie devs face this with cheap knock offs on google play store even. And all really successfull games will sooner or later face not so cheap knock offs that actually are not knockoffs but just imitate the concept. Looking at all the five nights at freddy "inspired" games.
All that being said, it is prudent to watermark some of your assets. For example with a link to you at a place where it is hard to crop out, if you have a banner. But I advise against certain technical measures, like a login server for an offline game. Even code obfuscation might hurt you more, than you would think, as it might trigger antivirus scanners. But that is more a problem of downloadable executeables.