This site allows you to create a encrypted html file (the container) which contains either a text message or an entire file. Once you chose a password, put in the text or file you want to encrypt and press "Generate Encrypted Container", the browser will create an HTML file which will then be automatically downloaded with your chosen name. That HTML file contains, in encrypted form, the entire message or file you wanted to encrypt. So you can then give that HTML file to someone and give him the password out-of-band or in any other way. They can then open the HTML file and put in the password if prompted. They either get the file then or the message. For encryption, this site uses xchacha20poly1305. And scrypt as a Key-Derivation-Function. Both implemented by Paul Miller. Of course, nothing you use here, neither password nor message or file, ever reaches me or my server.