Encrypt text in your browser without uploading it

A remote encryption service is a contradiction in terms. To encrypt something for you it has to receive the plaintext, and usually the passphrase along with it, so at the exact moment of use your confidential content is sitting in the clear on somebody else's machine. Doing it locally is not a preference about convenience; it is the only version of the operation that is coherent.

AES has never been the weak point, and it is not the thing to worry about. A short, memorable or reused passphrase is. Generate a long random one rather than inventing something you will remember, and accept that you have now traded a confidentiality problem for a key management problem, which is the real difficulty in every encryption story ever told.

Getting the passphrase to the other person is the part people fumble. The ciphertext and the passphrase have to travel by different routes, because anyone reading the single channel you used for both has everything. Sending an encrypted file and its passphrase in the same thread is the most common mistake here, and it undoes the entire exercise.

Verify by disconnecting the network before you type anything sensitive. Encrypt, copy the result, then reconnect. Nothing you typed, and no passphrase you used, was in a position to go anywhere, which is exactly the guarantee a hosted encryption form cannot offer no matter what it says.