The Solution using Standard Markdown ( not iFrame! )
Using an iframe is not the "obvious" solution... especially if the Markdown parser (or publishing platform) you are using does not support inlining content from a different website ... Instead you can "fake it" by including a valid linked-image in your Markdown file, using this format:
[](http://www.youtube.com/watch?v=YOUTUBE_VIDEO_ID_HERE "Video Title")
Explanation of the Markdown
If this markdown snippet looks complicated, break it down into two parts:
an image

wrapped in a link
[link text](http//example.io/my-link "link title")
Example using Valid Markdown and YouTube Thumbnail:

We are sourcing the thumbnail image directly from YouTube and linking to the actual video, so when the person clicks the image/thumbnail they will be taken to the video.
Code:
[](https://www.youtube.com/watch?v=StTqXEQ2l-Y "Everything Is AWESOME")
OR If you want to give readers a visual cue that the image/thumbnail is actually a playable video, take your own screenshot of the video in YouTube and use that as the thumbnail instead.
Example using Screenshot with Video Controls as Visual Cue:

Code:
[](https://youtu.be/StTqXEQ2l-Y?t=35s "Everything Is AWESOME")
Clear Advantages
While this requires a couple of extra steps (a) taking the screenshot of the video and (b) uploading it so you can use the image as your thumbnail it does have 3 clear advantages:
- The person reading your markdown (or resulting html page) has a visual cue telling them they can watch the video (video controls encourage clicking)
- You can chose a specific frame in the video to use as the thumbnail (thus making your content more engaging)
- You can link to a specific time in the video from which play will start when the linked-image is clicked. (in our case from 35 seconds)
Taking a screenshot takes a few seconds and there are keyboard shortcuts for each OS which copy the screenshot to your clipboard which means you can paste it for even faster upload.
Not Only C#
And since this is 100% Standard markdown, it works everywhere (not just for the C#
parser!) ... try it on GitHub, Redit or Ghost!
Vimeo
This approach also works with Vimeo videos
Example

Code
[](https://vimeo.com/3514904 "Little red riding hood - Click to Watch!")
Notes: