fix(html reporter): toggle visibility in IE11

[gitgrimbo]

Fixes #927

[codecov-io]

Codecov Report

Merging #930 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #930   +/-   ##
=======================================
  Coverage   92.45%   92.45%           
=======================================
  Files          58       58           
  Lines        4624     4624           
  Branches     1012     1012           
=======================================
  Hits         4275     4275           
  Misses        349      349

Impacted Files	Coverage Δ
src/lib/reporters/Html.ts	86.81% <100%> (ø)	⬆️

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