It will determine how anchors are created and will break existing
links otherwise.
Adapted from Revert "Make `user-content-* ` consistent with github (#26388)
(cherry picked from commit 1666fba8f5)
(cherry picked from commit 48f38280e8)
(cherry picked from commit 03adb3a2b4)
(cherry picked from commit a0ad36f0ad)