Updated for Sparkle 2.2
You can add a new video to the page by picking the video element from the More toolbar button. If you plan on using a video file from your local disk, a shortcut is to drag an mp4 file in from the Finder, which will be turned into a video player for that file.
The video element works with in two main ways, based on the video source.
To substitute the selected placeholder with a video, you can use the dropdown menu at the top of the Style pane of the Inspector to choose:
For videos from YouTube and Vimeo there are options for allowing autoplay and the offering of related videos, a menu to choose various types of Embedding, and the ability to add a Custom Poster Image, i.e. a preview frame more of your liking.
For videos which have been dragged and dropped, added From Disk or From Network Location, it's possibile to pick a Poster Image from the video or add one, enable Autoplay, Mute, Looping, show Dark or Light Player Controls and also choose some Embedding options.