Just in case anyone find this post like me looking for a way to add HTML to their markdown page, I ended up doing the following…
Create a custom shortcode called something like raw-html.html with the contents:
{{ .Inner }}
Then wrap your HTML in your content like so:
{{< raw-html >}}
<video width="480" height="480" autoplay muted loop playsinline>
<source src="https://i.giphy.com/media/687qS11pXwjCM/giphy.mp4" type="video/mp4" />
Your browser does not support the video tag.
</video>
{{< / raw-html >}}
Not saying this is a good idea or solution, but it seems to work.