Nice things with SVG

One fun thing that can be done with SVG files: you can use entities in an inline DTD to define constants to be shared across different places in the file. You can see some great examples of this in the SVGs in David Ellsworth's "Squares in Squares" page [0].

The major browsers have no issues with this, though note that some tools like Inkscape won't parse the DTD nor expand the entities.

[0] https://kingbird.myphotos.cc/packing/squares_in_squares.html

You say "entities" but that term is actually the name for SGML/XML's mechanism to define arbitrary syntactic content for reference/reuse with entity references a la &ref, whereas in SVG you can park shapes/paths/whatever under refs, giving those an id attribute value, and then <use> those element in the body SVG content, which is also what the page you linked is using (for each individual SVG ie. there's no sharing of rectangles across the many pictures since these are pulled-in individually via <embed> inot their own DOM rather than used as inline SVG).

I wonder why SVG's original designers found it necessary to supply an ad-hoc re-implementation of the entity mechanism. I think it might have to do with how rendering properties can be overridden at the usage site? At least I don't think it was established that browsers ignore entity definitions or basically anything in the document prolog/DOCTYPE considering SVG was part of W3C's push to replace HTML's SGMLish legacy syntax with XHTML/XML.