In September 2015 thread "Custom XML Export," Hans instructed  us on how to create custom tags. The example he presented is:



         \this{that}  \input ward

When that example is run with the current (2018-06-25) beta or TL18, there is no html body:

        <div class="document" xmlns="">

<div class="warning">Rendering can be suboptimal because there is no default/fallback css loaded.</div>

 <div class="metadata">


Further experimentation suggests that \startelement is gobbling the output. (Adding \setupstructure does not change the result.)

What is the proper way to add custom tags?