CCS save function in forms

Hi, does anyone know or used CSS for a save function in forms, which allows customers to save their data before submitting so they can come back and complete the form as and when they choose. Particularly helpful on longer forms, where it can't all be completed in one go. I've spoken to SquareSpace support and they say this function doesn't exist, but I'm sure there must be some coding for it. Sorry, bit of a learning novice on the CSS front, but can get by with clear instructions and some cut and paste code. Thanks again