This is great! It's informative enough to end users when a glyph is missing, and stands out for developer eyes when a web font subset doesn't include particular characters.

I am a native Sinhalese (that ක is from it), and there aren't enough web fonts that cover those glyphs. If the system fonts don't cover those glyphs (Windows, Linux, and MacOS versions do, but many feature phones, treadmill browsers, etc don't). I'd love to see this font (which appears to be only sub 500kb) instead of those ugly boxes that don't convey any information whatsoever.