Don't trust, verify. Read the code. Less than 60 lines! No third-party code.
Use it offline. Download and open verifiable-file-encryption.html in any browser on any device.
The encrypted file is indistinguishable from a random sequence of bytes (there are no identifiers).
Prefer passphrases over passwords because they are easier to remember and have higher entropy. 6 words have over 128 bits of entropy in wiktionary-passphrase-generation.html
Details: AES-256-GCM. Key is SHA-256(passphrase). Encrypted file is random 12-byte IV + ciphertext + 16-byte tag. Uses window.crypto browser API (no homemade crypto).
Maximum file size: ~64 GiB (NIST SP 800-38D sec. 5.2.1.1)