Add videos to wiki pages using the video-plugin which supports popular video services.
The Video Plugin embeds a video player on a wiki page. The video itself must be uploaded to a supported video site and the key from that service used to configure the plugin. Drag and drop makes this easy in some cases.
Beware Older Instructions
We are no longer lenient about arbitrary html in wiki pages. You may encounter older paragraphs that include service supplied <iframe> style embed codes which now show as a jumble of tags. These contain within them the service provider's identification key. You can add that to a video plugin to show the video.