Timeless does not include the tagline (site subtitle), unlike the other skins available on Wikimedia wikis. In those skins, this is an element with id siteSub (except for Cologne blue, which uses an element with class tagline). Though hidden on many sites, it is shown on English Wikipedia, and in print views for some skins.
See also https://www.mediawiki.org/wiki/Manual:Skinning_Part_2#Tagline
Fixing this would resolve T191589: Ensure compatibility with the metadata gadget (which displays an article assessment not shown in Timeless) without any changes to the gadget itself.