Maybe I am missing something, but can't find any !doctype or !element that would represent a DTD on that page. If you are talking simply about SVG defs and use - that isn't a DTD.
They're all in the standalone files, e.g., look at the source of https://kingbird.myphotos.cc/packing/square-11.svg. It defines a bunch of entities to represent various lengths and angles, which obviously can't be <use>d. (CSS variables would be an alternative these days, but many non-browser tools such as librsvg will have trouble with those as well.)
Aah thanks - these don't show up in the chrome inspector as by that time they are apparently already substituted.
