Just a 'me too'. No real solution. So far, disabling all my Safari extensions (1Password and Better) seems to help.
iMac late 2009, 27", 2,8 GHz i7, 20 GB, 1 TB HDD, ATI Radeon HD 4850 512 MB
macOS Sierra 10.12.2
When using Safari the iMac suddenly stops responding to keyboard and mouse clicks. The cursor can still be moved though. The iMac seems to still be running but the GUI is frozen. The only solution is to power it down with the hardware button on the back. It usually happens after loading a new web page and I usually notice when I start scrolling down and nothing happens.
For some of the freezes there is a 'watchdog' entry in the logfiles that others have mentioned here, but not for all freezes.
The problem also occurs on a second (non admin) account on the same iMac.
I did a clean install of 10.12.2 from El Capitan. The problem occurred soon after this clean install, before I had copied over my user data and installed my applications, extensions etc. again. I have had this problem before on previous versions of OS X on the same iMac, but not on El Capitan and never as often as now. I also have a MacBook (unibody) late 2008 on El Capitan that has no problems with Safari.
I have had contact with a senior advisor from Apple Support. He made me disable plugins and extensions in Safari. So far that seems to prevent the problem. I've enabled plugins again without a problem (I have only one plugin installed: Citrix Receiver). However, as soon as I enable at least one extension (either 1Password or Better) the problem reoccurs. Sometimes within half an hour and sometimes after several hours of using Safari.
I've done some testing in Safe Mode but only for 45 minutes or so. The problem did not occur in those 45 minutes of Safe Mode.
I have not tried to disable hardware acceleration in Safari yet.
The problem doesn't occur when using Opera instead of Safari.