From https://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html#9.e: > Decimal numbers must begin with a non-0 digit, octal numbers with a 0 > digit, and hexadecimal numbers with a 0x prefix to numbers and > hexadecimal letters a-f or A-F. Fixes https://github.com/fonttools/fonttools/issues/1541