Rounded images inside README.md files?

I’m wondering if it is possible to do rounded images inside a README.md file? Perhaps there is a permitted CSS classname we can add?

As doing what GitHub does:

<a class="d-inline-block" data-hovercard-type="user" data-hovercard-url="/users/balupton/hovercard" data-octo-click="hovercard-link-click" data-octo-dimensions="link_type:self" href="/balupton"><img class="avatar avatar-user" src="https://avatars.githubusercontent.com/u/61148?s=80&amp;v=4" width="40" height="40" alt="@balupton"></a>

Just turns into the following:

<a href="/balupton/readme-rounded-images/blob/main/balupton"><img src="https://avatars.githubusercontent.com/u/61148?s=80&amp;v=4" width="40" height="40" alt="@balupton" style="max-width:100%;"></a>

Source repo: readme-rounded-images/README.md at main · balupton/readme-rounded-images · GitHub

GitHub’s template CSS does not apply to how markdown (or other supported markup formats) are rendered in GH WebUI. HTML tags in markdown are stripped of any CSS, scripts, etc., only basic and bare tags make it through the rendered document (for security reasons).

But even with other supported markup syntaxes that support class names (e.g. AsciiDoc), these are not reachable from the document preview on GH — i.e. the preview docs are “sand-boxed” (so to speak) from any CSS that is not strictly part of the intended preview-template.