iMac dies after Safari update

iMac cannot boot after installing Safari update.

There is a new Safari update. After I installed it, my home iMac cannot reboot.

For about an hour, the iMac was stuck in the grey Apple screen (sometimes with, and sometimes without, a progress bar). The progress bar would “finish,” then the Mac would restart back into the grey Apple screen.

I decided to leave the iMac alone while I worked out.

When I returned two hours later, the iMac had progressed from the grey screen to a blue screen. Zapping PRAM and restarting gets me a grey Apple screen followed by a blue screen. The blue screen flashes twice while changing its color balance settings, indicating that some part of the operating system still works. Then a tiny white rectangle appears at the top left of the screen. Then, nothing further.

This machine contains my entire music collection and all my photos of my daughter.

I have a backup drive but cannot force the iMac to boot from it.

I have an OS X CD but cannot force the iMac to start from it.

Maybe this is what Apple means by “HTML5.”