German characters on the Mac (umlauts ¨ and ß)
For those of you interested in typing your answers out without using the mouse and generating these characters for use in questions posted like this one, here's a handy guide (for those of you using a Mac, with the North American keyboard layout).
The option key is used to generate special characters. Option-u will add an umlaut to the character that you type next. Option-s can be used to generate an ß.
Option-u, u: ü
Option-u, o: ö
Option-u, shift-a: Ä
This mechanic can be used for many other accent characters in other languages. You can see all possible characters by using the Keyboard Viewer and holding down modifier keys: open up Keyboard Preferences in System Preferences, under Keyboard, check "Show Keyboard & Character Viewers in menu bar", and then choose "Show Keyboard Viewer" from the menu that will then appear at the top-right of your screen.
It's actually not Safari--that's Mac OS X's built-in spellcheck. It figures out that you're writing in German and then identifies errors. So it's not actually correcting for umlauts per se--it's just noticing that you've typed a word that doesn't exist and suggesting umlauts to match a dictionary word.