mass upload of photos to iCloud photos on iCloud.com

In Safari on a Mac you can select several image files at once - in the file chooser dialog open the folder, then hold down the command key ⌘ while clicking the photos you want to upload, or enter ⌘A to select all at once.

Perhaps there is a similar way to select several items in other browsers on other system versions.

you cannot select folders for upload, only multiple images. to upload folders you have to sync with-a photos library on a mac.