Fix/htmlproofer

[dweinholz]

Try to fulfill the following points before the Pull Request is merged:

• Please create your PR against the dev branch.
• Please wait for all tests to finish.
• All tests relevant to the changes introduced by this PR must pass. If a test fails due to an issue unrelated to this PR, it does not block this PR and should be handled separately.
• When adding new documentation or a tutorial, make sure the corresponding Markdown file is linked in config.yml.
• Once all the above points have been successfully completed, the PR can be merged.