We also already have the ?preview-style=default escape hatch as well.
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Good luck ESPing that to people having issues though 
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Reading through this it seems like the consensus is that both CSS and HTML customizations should be disabled, but still only CSS customizations are disabled. What’s the reasoning for that?
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Yup. Agreed with @AdamCapriola. Please remove all or none, and allow us to have a functioning admin.
And as @riking said…
We are all adults here and can swim without floaties. Well… most of us can.
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Can you please provide some more detailed steps as to how to perform this?
Seems like I have this problem only with bad javascript that I ncluded in the head/ and now breaks quite a lot in the site including the save functionality in the head/ section.
Have you tried this: Enter your forum via Example Domain, as quoted above?
I solved it after posting here: Admin panel not working after including buggy js in head/