Hi Oleg, you are right, BUT
since xml-sitemaps crawler anyway crawls the site, it is easy to output those info into another file, named "manifest.appcache" , file that could be manually referenced in document's html tag.
<html ... manifest="manifest.appcache">
With the same crawl ( no extra job load on the server ), site will load faster because updated files ( .css .js. .json .jpg .jpeg .phg .webp , etc ) will be loaded and cached when opening the page.
latest improvement with hreflang is AWESOME, specially for EU-multilanguage sites.