A faster and more convenient way: We associated each character with a number from 1 to 4, whereas ä is 1, ö is 2, ü is 3 and ß is 4. Just type in the number and it will be instantly transformed to the character.
Enjoying higher usability
Use the tab key to jump from the current to the next input field.
Press enter key (or return key) to send the form and to see the solutions.
Use numbers 1 to 4 to add vowels and special characters.