I'm trying to create an input button that will link to a .css file that is created at the time the visitor clicks the button.

For example. There is a text area with css code in it that the visitor edits then when they click the download input button and they are prompted to download a .css file that contains everything from the textarea.

is this possible?

I'd say you can't do that using javascript: you can submit the data to a server and the server can force download.

Not with javascript, js is a client side language which doesn't have the ability to read/write on the file system. To do what you are asking you need to write the css the user has typed to a file. Can't do it in js however this can be easily done in PHP or some other server side language.