Azure AD B2C strips html tags from Custom UI template

While trying to customise the unified (Sign In & Sign Up page), I have this simple HTML in my unified.html template (fragment):

<div class="col-4 login-box gradient-background">
  <div>
    <h1>WELCOME TO<br/>SuperFancyProductName<sup>&reg;</sup></h1>
  </div>
  <div id="api" data-name="Unified"></div>
</div>

However, when Azure AD B2C renders the Sign In page, the element is stripped of from inside the h1 element, with this result (fragment):

<div>
  <h1>WELCOME TO<br>SuperFancyProductName®</h1>
</div>

In our case, this does not allow us to properly align the ® symbol.

Is there any documentation on what tags are allowed in the template html and how this template transformation actually works?