Hi jonter, interesting issue and appreciate the simplified code example - but (like rainborick) I can't reproduce the issue at all. I don't Chrome, but on the provided code winSafari5 doesn't produce horizontal scroll bars without adjusting the code, and when it does, the word "content" remains aligned as desired. That makes me wonder if there is something in the original code that is affecting the alignment.
Being unable to see the exact issue makes it hard to suggest a "fix", however I did achieve horizontal scroll-bars and the correct alignment by setting 900px as the min-width on HTML (not body). My other thought was exploring to see if background-size might help.