In https://gerrit.wikimedia.org/r/c/mediawiki/skins/Vector/+/803941 we broke functionality that allowed extensions/gadgets to mark conntet in such a way it didn't overlap the header.
- Restore the CSS
- Improve the documentation so it's not accidentally removed again
- Go to https://en.wikipedia.beta.wmflabs.org/wiki/Sticky_test and login. Scroll the page until the "sticky element" text sticks. The sticky text should not overlap the sticky header