This one is interesting. Looks like they don't send any requests for username. For email address, they have a delay before sending to the server to see if it'

This one is interesting. Looks like they don't send any requests for username. For email address, they have a delay before sending to the server to see if it's a used email address (if you type quickly enough, it will only send a single request to validate the email). For password, they start sending every character you type once the field has 6 characters in it. It then sends your full form details on every keypress (plus it has a delayed send to the same password_strength call, similar to what they do for emails). So if you type your password slowly, it will send your details twice for every keypress.