Are you using Firefox? I see the same black box in Firefox too (which is actually an SVG inside an <img> tag), but in Chrome, it renders fine: https://imgur.com/a/NdvKN
It looks like the SVG's style block[0] applies a black background to the svg element:
svg {background: rgb(1.0,1.0,1.0)}
which fails to render in chrome due to the floats but renders fine in firefox. If set to white or removed entirely, it will display correctly in Firefox.
0: http://beautifulracket.com/img/polboxes16.svg