As @mofosyne pointed out in the strikethrough topic, there is a difference between <s> and <del>.
I would like to review my earlier comment about ~~this text is deleted~~ being used for <del>. It actually makes more sense to use -- since minus is the opposite of plus and ++ is the proposed <ins> syntax. ~~ could then be freed up for <s>. I don’t think -- would clash with horizontal rules since these must be at least three characters long. Unordered list items are one character wide, so no conflict there. There is the danger of conflicting with em dashes and en dashes however, so we should consider these.