Fix hidden element visibility

[tvdeyen]

What is this pull request for?

We recently made sure that all HTML elements with
the hidden class do not display at all. But we want to show elements that are hidden, but dim them. Using a dedicated element-hidden class instead.

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have added tests to cover this change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.88%. Comparing base (659a96c) to head (8e098ee).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2799   +/-   ##
=======================================
  Coverage   95.88%   95.88%           
=======================================
  Files         223      223           
  Lines        6082     6082           
=======================================
  Hits         5832     5832           
  Misses        250      250           

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