How do I add videos to a gallery?

Films and clips go in beside the photos, the same way. The couple and their guests watch them in the gallery and download them with the photos.

Open the gallery on its Photos tab.
Press Add photos and pick the films, or Add a folder, or drop them anywhere on the page, just as with photos. A film inside a folder joins that folder's section.
Each film gets a still for the grid and the cover, taken about a second in, then goes up in parts. The card in the corner shows how much is sent. A long film takes a while: keep the gallery open, and move to Design or Settings meanwhile if you like.
On the Photos tab a film shows ▶ and its length. Its ⋯ menu works as a photo's: Make cover uses its still, and it can be moved, made a highlight, duplicated, hidden or deleted.

How films play

Nothing plays by itself: in the gallery a film is its still, with ▶ and its length.
Opened, it plays with sound in a player in the gallery's own colours and type: play and pause, a time bar, sound and full screen. Where a browser won't start with sound, it plays quietly and shows Tap for sound.
It streams as it plays, so a long film starts quickly and can be skipped through. Going on to the next photo stops it.
On a computer, Space plays and pauses, M turns the sound on or off and F is full screen. The arrow keys skip five seconds on the time bar and in full screen; otherwise they still go from photo to photo.

A film has no smaller version: whatever sizes you allow under Downloads, it downloads as the file you uploaded.
It comes on its own from Download at the top of the viewer, which says its size, and in every ZIP (the whole gallery, a section or the favourites) whenever downloads are on.
On a phone, Save puts a film of up to 250 MB in the photo library; a longer one downloads to the phone's files. The sets of ten under Save to your phone's Photos leave films out.
Activity counts them apart: “1 film”, or “486 photos · 3 films”.

Formats and limits

What	Details
Files	MP4, MOV and M4V
Size	Up to 10 GB each, counted in your storage like an original
Plays in every browser	H.264, in an MP4 or a MOV
May not play everywhere	HEVC (H.265) and ProRes: they go in and download, but some browsers can't play them

A film shows a plain grey still: Your browser couldn't open it to take a still, often because it's HEVC or ProRes. The film went in all the same. For a real still and the widest playback, export it as H.264 MP4, add it again and delete the first one.
A guest says a film won't play: Their browser can't play the film's format. The player tells them so, and offers the download when downloads are on. An H.264 MP4 plays everywhere.
The upload says a film is too large: Films can be up to 10 GB. Export it at a lower bitrate, or at 1080p instead of 4K, and add it again.