Impressive!

[KenyOS]

I like a lot of the design how you built this, so simple indeed. I was about to study Web Cryptography API can be a good start with your project. I have a few question...
How strong is the cryptography used on the example page?
I guess that if someone use bruteforce method we should garantee a strong password like 12+, right?
it's nice that support files too, but how much I can store inside? kb? mb? gb?

[mprimi]

Thank you, Keny!

How strong is the cryptography used on the example page?

Encryption is AES-GCM which is a NIST-recommended standard.
Using a password directly as key for AES is not a good idea, entropy would be way too low.
This is why the password is transformed into a key using PBKDF2.
Still, if someone chose a very simple password (like banana in the examples) the output is trivially crackable using dictionary attacks.

The bounty example is I think closer to what I'd recommend (3-5 words).

But it also depends on your use case. If you want to send an email, and all you care about is Google not reading it, then even using 1234 as password, and including the password itself in the hint would still get the job done.

The file size depends on the browser. Specifically the (data URI)[https://en.wikipedia.org/wiki/Data_URI_scheme] generated upon successful decryption. I think for most browsers is a few hundred MBs. But admittedly I have not tried anything bigger than a few MBs.