Improve HTML Report formatting

[coliff]

• Ensure valid, semantic HTML
• Nicer fonts and formatting

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (95e6a13) to head (8121e6e).
Report is 43 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #1617      +/-   ##
===========================================
+ Coverage   96.99%   100.00%   +3.00%     
===========================================
  Files           2         1       -1     
  Lines        1628         1    -1627     
  Branches      335         0     -335     
===========================================
- Hits         1579         1    -1578     
+ Misses         49         0      -49     

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