Turn on the browser setting to save passwords
Chrome: go to chrome://settings/, show advanced options and check "Offer to save passwords I enter on the web"
Firefox: go to Preferences/Security and check "Remember passwords for sites"
Safari: go to Preferences / Passwords and check "AutoFill user names and passwords"
Enter username="someUser" / password="somePass" and hit submit
There should be a popup that asks whether you want to store the password
The next page should show the values in json format
Go back
Try and detect autofill via webkit only hacks
User
Password
Submit
Reset
Clear