Wikipedia supports HTML comments. You can enclose text in <!-- and -->. This means that while the text will still be visible in wikisource, it will not be visible on the displayed page.
I see in many articles that people add comments like: "To do: .." or "needs to be expanded" or "who was this?". Some pages also have "Notes to Wikipedia editors" and the like.
In case of detailed discussions, this should of course be moved to the Talk page. But if it is useful to have directly in the text but only useful for editors, we should use HTML comments for these kind of meta- remarks. A non Wikipedia editor should not be exposed to meta-content like this.
So when you see such a remark and don't want to delete it, please enclose it in <!-- and --> to hide it from the rendered page, but not from those who edit it. You *must* use this exact character sequence, the shorter <!- foo -> or other variants like <- foo -> will *not* work.
Thanks!
Erik