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

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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