There IS in fact a way to mass delete photos in the iCloud Photo Library web interface.

1. Click 'Select Photos' at the top
2. Select the very first photo
3. Scroll all the way to the bottom
4. Hold the SHIFT key and click the last photo (this will select everything in between)
5. Click 'Delete' at the top

There IS in fact a way to mass delete photos up to 1,000 at a time in the iCloud Photo Library web interface.

1. Click 'Select Photos' at the top
2. Select the very first photo
3. Scroll all the way to the bottom
4. Hold the SHIFT key and click the last photo (this will select everything in between)
5. Click 'Delete' at the top