IFrame embedding does not work

Summary

I want to embed an IFrame in my README file and it does not work: the <figure> element is created but the iframe is stripped.

My use-case is showing build information by some Gitlab pipelines artifacts inside an Iframe. The .. raw: html directive in reStructuredText does not work either.

I used the code from https://about.gitlab.com/handbook/markdown-guide/#display-videos-from-youtube . I know that's a different engine than used in issues and previews. Still, embedding content is something that should work.

Steps to reproduce

This is the code from the handbook docs:

<figure class="video_container">
  <iframe src="https://www.youtube.com/embed/enMumwvLAug" frameborder="0" allowfullscreen="true"> </iframe>
</figure>

Now we try to render it:

What is the current bug behavior?

As can be seen, no video showed up above.

What is the expected correct behavior?

I would expect a video to show up just as described in the docs. If you inspect the generated HTML you can see the <figure> element made it but the <iframe> was lost.

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

N.B. For Figma specifically, GitLab have released a plugin to upload designs to issues directly