disconnect all USB devices to see if POST can proceed without stalling.  this includes all USB keyboard, mouse etc.  If you have a multi media card reader that connects to mobo internal USB port, disconnect it too.  If you have any wireless USB or bluetooth device, remove it.  you have shown it is not due to HDD or Windows.  Some hardware/firmware is not passing POST.
Here is what seems to have caused this:  Computer was taking a long time after startup from sleep to recognize the usb keyboard even though I just purchased a new Dell keyboard.  Seemed to work ok a few weeks ago so I thought I would try a recovery back to 9-15-2021.  Did this and then it seemed to work a little bit better.  This also seemed to put Windows Update into Manual mode.  Clicked on Check for Updates just to see what was removed...Windows decided that I must want to update.  It said it would update on the next restart.  But since I didn't want to update I changed to Windows Update Disabled via the services.msc run command.  Then I just selected Shut Down, rather than Update and Shut Down.  The next day when I rebooted the pc it hung on the Dell Logo. Things I've tried:  Power up and press F2, F8, and/or F12.  No difference. Put in Windows 10 installation DVD and powered up.  No difference. Put in Windows 10 installation USB memory stick.  No difference. Force power up/power down multiple times.  No difference. Disconnected the HD, put in Windows 10 installation DVD, and powered up.  No difference. Reset the CMOS.  It took a little longer before the Dell logo appeared but otherwise no difference. Any other ideas?
XPS 8910, stuck at Dell logo during startup