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I'm developing an OpenType font by byte-crafting it rather than using design tools, and now have a custom OpenType (CFF flavour) font that implements a really small subset of the Basic Latin unicode block (specifically it only implements .notdef and what under Basic Latin interpretation would be the tilde).
The CFF block passes "tx -3" validation, the font as a whole passes a round-trip de/recompile through TTX, and Microsoft's Font Validator does not report any errors in the font. Even loading it as a custom webfont (as plain otf in browsers that support otf, and wrapped as WOFF in browsers that don't) makes them style the implemented ~ glyph correctly.