Text rendering: Support subpixel glyph placement
Gtk 3.x or Cairo so far renders glyphs for a certain size once and reuses the bitmaps for every other instance of the same size. This makes inter-glyph spacing slightly irregular.
Supporting subpixel-placement requires keeping several copies of a glyph bitmap in memory, unless you rerender everything on the fly. Qt4/5 support it, Chrome (or at least the Electron framework used by Atom) supports it.
http://www.antigrain.com/research/font_rasterization/index.html shows an example, scroll down to the first "A quick brown fox jumps over..." image.