syncthing to sync your notes folder (and your pictures, audiobooks, whatevers); then the editors can be anything, i.e. whatever you prefer on each platform. sync over the internet can be disabled and set so it works only on your home LAN. also, sync direction, maybe you want it to sync only one way. turn on versioning to get at the version from two weeks ago. etc.

you should be in the know by now to not get locked in by any one provider or app. always go with the thing that gives you exit options.

Signal has a Note to Self. Bonus; encrypted.

Could I ask why you need web app? There are plenty of great desktop applications that will work well, sync with your phone, and can be accessed when you don't have an internet connection.

Everybody knows Obsidian, but you might think that's overkill. It's also closed-source unfortunately. I use Marktext, which is open-source and more lightweight than Obsidian. I don't think it's developed anymore, but the core functionality works well enough.

You might also want to use VSCodium, as that also supports Markdown files, or perhaps some other open-source editor, like Kate!

As for Android apps, I can recommend Markor as it is open-source and supports plain Markdown files w/ folder structure and all. Obsidian is also available on Android if you want that.

For syncing, I like Syncthing since it's peer-to-peer (does not require a server, so devices just need to be on the same network to sync). You have a remote device, so server-based syncing will probably work better for you. In that case, I can recommend Nextcloud for that, it works well and has clients for all the different platforms. You can self-host it or use one of the many public instances! There are other options too, like OpenCloud and Seafile, but I haven't used those myself. You could also use another cloud storage provider, there are plenty of those that offer sync features.

I use Memos which comes with a built in web client. For an android client I use MoeMemos (it also has a local offline mode if you don't want to connect to Memos). I've had trouble with my MoeMemos app logging out and I have to generate a new access token to replace the old one, which is annoying but doesn't happen often.

I used simple note for a long time and enjoyed it. Did have some occasional sync conflicts resulting in lost notes, which was frustrating. Now I'm hosting a Joplin server and really enjoying it. I'm not actually sure if Joplin can be used in a browser, but to my knowledge it has apps for every OS.

https://turtlapp.com/ would be a nice middle ground.

I've been using https://simplenote.com/ for some years now. Works on all my devices

How 'bout Nextcloud Notes?

You'd need a Nextcloud server though (either one you run, or someone else's that you have an account on). That gives you a web UI too, in addition to the Android app.

Standard Notes. It's now owned and run by Proton (as in Proton VPN).

Google Keep Notes?

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Joplin: https://joplinapp.org/

But:

Hmmmm I was gonna say they have no web app, but it appears they do:

https://app.joplincloud.com/

Jotty: https://jotty.page/

But:

It's self hosted, and has no android app. You can do the browser app thing tho

Anpther solution (but no web app) is signal which can send a message to yourself. That's whay I use when I just need to get a url or something like that over quick.

I use markor. Sync with syncthing. I think it can do webdav

Crypt.ee