I had a droll experience with ebooks a few years ago. Some printed books come with a publisher's advertising insert listing their other current offerings. When the book itself is old enough to be in the public domain, this information is interesting in its own right,* so we generally retain it. I typically put it inside a div with a transparent name like "pub_ads". Well, some of those names were a little too transparent, because I noticed that my browser wasn't displaying this section :) Oops.
Now I give advertising sections a less obvious descriptor. But I keep comment lines like <!-- ads begin here --> because ad blockers don't seem to heed those.
* Especially when you have the fun of making half the listings into active links in the form "example.com/ebooks/12345/" and the other half can be flagged as "in preparation".