I use iCloud Photo backup for my photos. It's convenient. I can hold many thousands of photos and videos without overwhelming my iPhone memory storage.
It keeps a low res version of the photos, and it download the his-res only when necessary (e.g. when editing). For videos, it's keeps a preview, and downloads the video when I request it.
However, I would like to access specific albums, containing both photos and videos, even while I'm offline.
Is this possible? How?