But what happens when javascript grabs that input

Could you be more precise? A function that reads from the socket should return bytes. A function that returns the text of an XML node should return decoded chars. It all depends on the interface.

It's a question that's easily answered by trying.

when a user enters data into a form

Just a quicky to hold you until someone comes along with more info...

This is very rough area. IIRC, the agent is suppose to encode the data using the same encoding as the page on which the form resides. However, I remember having a discussion about how it isn't well supported and/or there are issues with the approach.

Of particular interest is that some browsers (possibly all the major ones) will populate a specific field with the encoding when the field is provided in the form. I can't remember what the field is called.