Amusingly, here it is also BitMap [1]. Why they use an obsolete noncompressed proprietary format instead of PNG I don't know.

Edit: looks like it's because BMP supports 1-bit packed pixels and ~~PNG doesn't~~ (Edit to edit: this is wrong). The file sizes are almost identical; the 8x difference in the number of bits is exactly balanced by PNG compression! On the other hand, PBM [2] would've been a properly Unixy format, and trivial to decode, but I guess "the browser knows how to render it" is a pretty good argument for BMP. macOS Preview, BTW, supports all the NetPBM formats, which I did not expect.

[1] eg. https://unifoundry.com/pub/unifont/unifont-17.0.03/unifont-1...

[2] https://en.wikipedia.org/wiki/Netpbm