most (not all, some contributions are QTVR-only due to historical reasons)
many, but depends on photographer's choice of file upload (newer event = more likely to work) and your device's capabilities. You may experience slow playback, distorted images or unexpected browser quitting if there is only limited memory/GPU power available.
This simple panoramic photograph was taken at the top of the Astoria Column, thanks to Betty for the tip (she knows who she is!) – I would have not found this spectacular spot had it not been for her.
Looking north across the Columbia River (the river with the large ships) is the state of Washington, looking west is the Pacific Ocean.