[NTG-context] Custom tags, revisited

26 Jun 2018

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

    \setupbackend[export=yes]

    \definehighlight[this]

    \starttext
         \startelement[what]
              \this{that}  \input ward
         \stopelement
    \stoptext

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

         </head>
         <body>
             <div class="document" xmlns="http://www.pragma-ade.com/context/export">

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

    <div>
      <div class="metadata">
      </div>
    </div>

             </div>
         </body>
    </html>

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?

-- 
Rik