Well, we have plenty of images already: <img>, <picture>, <figure>, <object>, <iframe> can be used for that purposes too. It is just unclear how <icon> is different from <img>.

As of <include> (so called client side includes) Sciter has them [1] as they make sense in precisely desktop environments - when files included from local sources (synchronously).

[1] <include> in Sciter: https://sciter.com/forums/topic/import-html-in-html/