Fix two sources of memory leaks in JS and CSS rewriting

[benoit74]

Fix #322

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.96%. Comparing base (b1e3b08) to head (a5195e8).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #323   +/-   ##
=======================================
  Coverage   99.96%   99.96%           
=======================================
  Files          41       41           
  Lines        2554     2558    +4     
  Branches      364      364           
=======================================
+ Hits         2553     2557    +4     
  Misses          1        1           

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