I made a small edit of the post https://stackoverflow.com/revisions/25855713/5 that showed steps and sub steps as html headers, and noticed that after my change the steps start to be shown as plain text, e.g. #Step 1 - Understanding Basic Authentication.
I decided that it was caused by my changes and reverted to previous revision 4 created in 2016. Unfortunately the formatting wasn’t restored.
I am guessing that since 2016 Stackexchange replaced its markdown interpreter and stopped recognise # without spaces before text as headers. In the markdown documentation https://daringfireball.net/projects/markdown/syntax#header space after hashes is shown, but not documented and some implementations enforce space, but other do not enforce.
I believe that old html was cached and showed headers correctly before I touched the post.
If my explanation is correct, it will be good to run convertion to update all old posts:
For lines starting with 1-6 hashes without spaces before the text to insert single space after the hashes to make backward compatible support of hash headers.