Donors are getting used to clicking on a "Donate" link in an email and having their basic contact information automatically filled on out the form. We've had several donors complain about how they don't like that they have to fill out all their info each time we make a donation. And we can monitor through Constant Contact that many people who go to our donation page don't complete their donations. Having an autofill is super important for donation conversion rates, especially now that people read emails and donate through their phones. (Typing everything out on a phone is annoying!) A cumbersome donation process means that people quit the donation page before they process their donation.

Check out this Salsa page explaining how they do it: https://help.salsalabs.com/entries/22543883-Autofill-supporter-information-on-forms