Excellent library - I used it to do some super fun things last year http://jon.gold/2016/05/robot-design-school/

By the way, OpenType.js has problems with parsing many fonts (old structures of TTF tables) and also with rendering some OTF (CCF) glyphs. If you are looking for a more versatile library, I recommend using Typr.js https://github.com/photopea/Typr.js

Typr.js doesn't seem to have a way to write out fonts after editing them. In what way is it more versatile?

You mean writing a font back into a font file? Yes, this feature is not available now.

OpenType.js throws errors for many font files. It supports just a specific subset of standard font structures. It also renders some glyphs incorrectly. Typr.js does not have these problems (at least not during my testing).

Neat. Although the rendered text isn't screen-reader accessible. An aria-label with the text content would solve this, though.

Definitely raise an issue: https://github.com/nodebox/opentype.js/issues