I'm going to plug my project PageCrypt here too, which predates StatiCrypt, has a web UI that's good for one-off page encryption, and does use the WebCrypto API.
PageCrypt is excellent. I wrote a tutorial a while back on Reddit about how to use it for password-protected Anki cards. Just to give that extra layer of security.
Yep, looks great! If I had known about it at the time I might not have written StatiCrypt. I added it to the "Alternatives to StatiCrypt"[1] section of the Readme years ago when I discovered it :)