Backstage not rendering <iframe> element in Docs

We’re using techdocs-cli to build our .md documentation files and publishing to S3. When viewing the Documentation locally using the spotify/techdocs Docker image our <iframe> renders. When we view our docs through a deployed Backstage instance though, the <iframe> does not render.

I’ve been looking into this and I believe it may have something to do with Shadow DOM. We have https://lucid.app as an allowed frame-ancestor in our Backstage app-config.yaml Content Security Policy, and I’m not 100% if that is passed to the Shadow DOM of our built documentation, or if it even needs to be passed?

Expected Behavior

When viewing documentation in Backstage, an embedded Lucid Chart <iframe> should render if it is allowed by the CSP.

<div style="width: 640px; height: 480px; margin: 10px; position: relative;">
<iframe allowfullscreen frameborder="0" style="width:640px; height:480px" src="https://lucid.app/documents/embeddedchart/xxxxxxxxxxxxx" id="xxxxxxx"></iframe>
</div>

Current Behavior

When viewing our documentation in Backstage if we inspect element on the Shadow DOM, our <div> is present, but the <iframe> is missing. We’ve verified in our S3 and the <iframe> is present in the index.html of our built documentation, but for some reason is getting stripped or cut out once it is in Backstage.

<div style="width: 640px; height: 480px; margin: 10px; position: relative;">
</div>

Steps to Reproduce

1. Write some .md file that has an html block similar to the <iframe> example above.
2. View that documentation locally through the spotify/techdocs Docker image
3. View the same documentation in Backstage (once docs have been built as html, and being pulled in as a Shadow DOM)

Context

Some of our teams have some complex Lucid Charts explaining their processes, infrastructure, etc. We would like to allow them to use Lucid’s embeddable <iframe> code for their documentation in Backstage.

Your Environment

Backend Backstage Dockerfile Base: node:14-buster-slim Edit: We are using release-2021-10-13

OS: Darwin 20.6.0 - darwin/x64 node: v14.8.0 yarn: 1.22.4 cli: 0.7.15 (installed)

Browser: Latest Chrome