From Halopedia, the Halo wiki

Video from YouTube can be shown on any Halopedia page using simple code. The video is not uploaded to Halopedia, but embedded. This means it is stored on YouTube, then called from there to be viewed on Halopedia.

Adding a YouTube video

To add a YouTube video, you need the ID code for the video. The easiest way to find this is to look at the URL of the YouTube page for the video. The string of letters and numbers after "v=" in the URL is the ID code. So for the URL: https://www.youtube.com/watch?v=-IhGS0OSdh4, the ID code could be -IhGS0OSdh4.

Check this out!
YouTube videos can be added using the following parser function syntax:

{{#widget:YouTube|id=VIDEOID|thumb|width=300px|caption=Check this out!}}

This style can be used in templates and supports passing parameters to the tag. The video ID is the only mandatory field; the rest are optional. Including the "thumb" parameter uses the thumbnail-style frame for video with the value for "caption" as its caption. The video will automatically resize to 16:9 with just the width provided, but the height may be provided if you require a different aspect ratio (such as 4:3).

For example, the video on the right is embedded with this code:

{{#widget:YouTube|id=DLFDzC_ok8c|thumb|width=300px|caption=Check this out!}}

If you don't want to use the thumb format, just don't include the "thumb" parameter. A "left", "right", or "center" parameter may also be used to align the video in each respected way, and a "height" parameter to specify the video height. The parameters can be placed in any order.

{{#widget:YouTube|id=DLFDzC_ok8c|center|height=400px|caption=Check this out!}}

Adding a Vimeo video

Vimeo videos can be added using the exact same formats as YouTube—just replace "YouTube" with "Vimeo" as the widget tag. Examples:

{{#widget:Vimeo|id=VIDEOID|thumb|width=300px|caption=Check this out!}}

These tags work the same way as described above.