Format HTML in the playground tab

[xxdavid]

Fixes #557 and also makes the HTML a bit nicer.

[xxdavid]

Before

After

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.45%. Comparing base (0b549e5) to head (81c5bbe).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #562   +/-   ##
=======================================
  Coverage   96.45%   96.45%           
=======================================
  Files          40       40           
  Lines        2201     2201           
=======================================
  Hits         2123     2123           
  Misses         78       78           

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