fix: compatibility for IE11

[zachguo]

fix #75

[codecov]

Codecov Report

Merging #76 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #76   +/-   ##
=======================================
  Coverage   96.89%   96.89%           
=======================================
  Files           3        3           
  Lines         129      129           
  Branches       31       31           
=======================================
  Hits          125      125           
  Misses          4        4           

Impacted Files	Coverage Δ
src/util.ts	90.62% <100.00%> (ø)

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[kerm1it]

Is it better to modify to document.documentElement

Can you try document.documentElement in IE11?

[kerm1it]

Trace in #81