Iframe

<iframe src="https://forms.justinjongstra.nl/f/YOUR-FORM-ID?embed=1"
        title="Contact" loading="lazy"
        style="width:100%;border:0;height:720px"></iframe>

Nothing runs on your page, so nothing can collide with your CSS and there is no script to audit. The cost is the height: an iframe cannot size itself to its content.

To fix that, add the loader and let the frame report its height:

<div data-fb-form="YOUR-FORM-ID" data-fb-isolate="iframe"></div>
<script src="https://forms.justinjongstra.nl/formbuilder/asset/embed.js" async></script>

The frame then posts fb:resize to the parent, which sizes it as steps change. The parent only accepts a message when the source window, the origin and the instance id all match. Checking the origin alone is not enough, since any frame from the same origin could otherwise spoof it.