Thanks to GitHub user aikrahguzar for telling me that #Emacs 30 added support for outline-minor-mode to eww (and more generally to shr output)! This lead me to learn about the outline-search-function variable that can be used to add support for outlines not based on regexps. I now updated Embark to also recognize that type of heading in general, and specifically in eww buffers. (The outline-search-function variable has apparently been there since Emacs 22, so I have no excuse for not finding it when I initially added support for outline heading to Embark. 😅)

[And on a personal level, aikrahguzar's information also allowed me to delete an entire little library (that added heading navigation and imenu support to shr) from my configuration! I *love* deleting code from my Emacs configuration.]