Custom OpenType / TrueType font not working and difficult to debug

I just used opentype.js to compile a font file sort of haphazardly. Here is what my final file looks like:

hexdump -C font.ttf
00000000  4f 54 54 4f 00 07 00 40  00 02 00 30 68 6d 74 78  |OTTO...@...0hmtx|
00000010  15 c3 8c 00 00 00 00 08  c3 b0 00 00 00 c3 b8 63  |...............c|
00000020  66 66 20 36 07 08 63 00  00 02 c2 a4 00 00 06 4a  |ff 6..c........J|
00000030  70 6f 73 74 00 03 00 00  00 00 02 c2 84 00 00 00  |post............|
00000040  20 63 6d 61 70 0e c3 b8  0e c2 a7 00 00 00 70 00  | cmap.........p.|
00000050  00 02 14 6d 61 78 70 00  3e 50 00 00 00 00 68 00  |...maxp.>P....h.|
00000060  00 00 06 68 68 65 61 00  01 00 01 00 00 00 44 00  |...hhea.......D.|
00000070  00 00 24 68 65 61 64 0f  c3 96 c3 be c3 bf 00 00  |..$head.........|
00000080  00 0c 00 00 00 36 00 01  00 00 00 01 00 00 00 00  |.....6..........|
00000090  00 00 5f 0f 3c c3 b5 00  00 03 c3 a8 00 00 00 00  |.._.<...........|
000000a0  c3 98 61 c3 9f 11 00 00  00 00 c3 98 61 c3 9f 11  |..a.........a...|
000000b0  00 00 00 00 00 00 00 00  00 00 00 00 00 02 00 00  |................|
000000c0  00 00 00 00 00 01 00 00  00 00 00 00 00 00 00 00  |................|
000000d0  00 00 00 00 00 00 00 01  00 00 00 00 00 00 00 00  |................|
000000e0  00 00 00 00 00 00 00 00  00 00 50 00 00 3e 00 00  |..........P..>..|
000000f0  00 00 00 01 00 03 00 01  00 00 00 0c 00 04 00 00  |................|
00000100  00 00 00 7e 00 40 00 05  00 3e 00 30 00 31 00 32  |...~.@...>.0.1.2|
00000110  00 33 00 34 00 35 00 36  00 37 00 38 00 39 00 41  |.3.4.5.6.7.8.9.A|
00000120  00 42 00 43 00 44 00 45  00 46 00 47 00 48 00 49  |.B.C.D.E.F.G.H.I|
00000130  00 4a 00 4b 00 4c 00 4d  00 4e 00 4f 00 50 00 51  |.J.K.L.M.N.O.P.Q|
00000140  00 52 00 53 00 54 00 55  00 56 00 57 00 58 00 59  |.R.S.T.U.V.W.X.Y|
00000150  00 5a 00 61 00 62 00 63  00 64 00 65 00 66 00 67  |.Z.a.b.c.d.e.f.g|
00000160  00 68 00 69 00 6a 00 6b  00 6c 00 6d 00 6e 00 6f  |.h.i.j.k.l.m.n.o|
00000170  00 70 00 71 00 72 00 73  00 74 00 75 00 76 00 77  |.p.q.r.s.t.u.v.w|
00000180  00 78 00 79 00 7a c3 bf  c3 bf 00 00 00 30 00 31  |.x.y.z.......0.1|
00000190  00 32 00 33 00 34 00 35  00 36 00 37 00 38 00 39  |.2.3.4.5.6.7.8.9|
000001a0  00 41 00 42 00 43 00 44  00 45 00 46 00 47 00 48  |.A.B.C.D.E.F.G.H|
000001b0  00 49 00 4a 00 4b 00 4c  00 4d 00 4e 00 4f 00 50  |.I.J.K.L.M.N.O.P|
000001c0  00 51 00 52 00 53 00 54  00 55 00 56 00 57 00 58  |.Q.R.S.T.U.V.W.X|
000001d0  00 59 00 5a 00 61 00 62  00 63 00 64 00 65 00 66  |.Y.Z.a.b.c.d.e.f|
000001e0  00 67 00 68 00 69 00 6a  00 6b 00 6c 00 6d 00 6e  |.g.h.i.j.k.l.m.n|
000001f0  00 6f 00 70 00 71 00 72  00 73 00 74 00 75 00 76  |.o.p.q.r.s.t.u.v|
00000200  00 77 00 78 00 79 00 7a  c3 bf c3 bf 00 04 00 04  |.w.x.y.z........|
00000210  00 04 00 04 00 04 00 04  00 04 00 04 00 04 00 04  |................|
00000220  c3 bf c3 99 c3 bf c3 99  c3 bf c3 99 c3 bf c3 99  |................|
*
00000280  c3 bf c3 99 c3 bf c3 99  c3 bf c2 9f c3 bf c2 9f  |................|
00000290  c3 bf c2 9f c3 bf c2 9f  c3 bf c2 9f c3 bf c2 9f  |................|
*
000002f0  00 01 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000300  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000370  00 03 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000380  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000390  01 00 04 01 00 01 01 01  04 66 6f 6f 00 01 01 01  |.........foo....|
000003a0  26 c3 b8 1b 00 c3 b8 1c  02 c3 b8 1d 03 c3 b8 1e  |&...............|
000003b0  04 c2 8b c2 8b c3 b7 5c  c3 a5 05 1d 00 00 00 c3  |.......\........|
000003c0  88 0f 1d 00 00 01 43 11  c2 8b 1d 00 00 06 4a 12  |......C.......J.|
000003d0  00 41 01 01 06 0b 0e 0f  10 11 12 13 14 15 16 17  |.A..............|
000003e0  18 19 1a 1b 1c 1d 1e 1f  20 21 22 23 24 25 26 27  |........ !"#$%&'|
000003f0  28 29 2a 2b 2c 2d 2e 2f  30 31 32 33 34 35 36 37  |()*+,-./01234567|
00000400  38 39 3a 3b 3c 3d 3e 3f  40 41 42 43 44 45 46 47  |89:;<=>?@ABCDEFG|
00000410  48 49 4a 4b 4c 30 2e 30  2e 30 66 6f 6f 20 20 66  |HIJKL0.0.0foo  f|
00000420  6f 6f 20 62 63 64 65 66  67 68 69 6a 6b 6c 6d 6e  |oo bcdefghijklmn|
00000430  6f 70 71 72 73 74 75 76  77 78 79 7a 41 42 43 44  |opqrstuvwxyzABCD|
00000440  45 46 47 48 49 4a 4b 4c  4d 4e 4f 50 51 52 53 54  |EFGHIJKLMNOPQRST|
00000450  55 56 57 58 59 5a 30 31  32 33 34 35 36 37 38 39  |UVWXYZ0123456789|
00000460  00 00 00 01 c2 8b 01 c2  8c 01 c2 8d 01 c2 8e 01  |................|
00000470  c2 8f 01 c2 90 01 c2 91  01 c2 92 01 c2 93 01 c2  |................|
00000480  94 01 c2 95 01 c2 96 01  c2 97 01 c2 98 01 c2 99  |................|
00000490  01 c2 9a 01 c2 9b 01 c2  9c 01 c2 9d 01 c2 9e 01  |................|
000004a0  c2 9f 01 c2 a0 01 c2 a1  01 c2 a2 01 c2 a3 01 c2  |................|
000004b0  a4 01 c2 a5 01 c2 a6 01  c2 a7 01 c2 a8 01 c2 a9  |................|
000004c0  01 c2 aa 01 c2 ab 01 c2  ac 01 c2 ad 01 c2 ae 01  |................|
000004d0  c2 af 01 c2 b0 01 c2 b1  01 c2 b2 01 c2 b3 01 c2  |................|
000004e0  b4 01 c2 b5 01 c2 b6 01  c2 b7 01 c2 b8 01 c2 b9  |................|
000004f0  01 c2 ba 01 c2 bb 01 c2  bc 01 c2 bd 01 c2 be 01  |................|
00000500  c2 bf 01 c3 80 01 c3 81  01 c3 82 01 c3 83 01 c3  |................|
00000510  84 01 c3 85 01 c3 86 01  c3 87 00 3e 02 00 01 00  |...........>....|
00000520  1f 00 31 00 33 00 45 00  56 00 67 00 7f 00 c2 94  |..1.3.E.V.g.....|
00000530  00 c2 a8 00 c2 b9 00 c3  91 00 c3 9c 00 c3 b5 01  |................|
00000540  0d 01 19 01 2c 01 3f 01  56 01 64 01 72 01 7d 01  |....,.?.V.d.r.}.|
00000550  c2 88 01 c2 a2 01 c2 b0  01 c3 81 01 c3 95 01 c3  |................|
00000560  a9 02 02 02 11 02 2f 02  46 02 5a 02 75 02 c2 8c  |....../.F.Z.u...|
00000570  02 c2 a3 02 c2 b6 02 c3  8f 02 c3 9b 02 c3 af 03  |................|
00000580  01 03 0c 03 1f 03 31 03  4a 03 56 03 65 03 72 03  |......1.J.V.e.r.|
00000590  7f 03 c2 92 03 c2 a3 03  c2 b4 03 c3 89 03 c3 9a  |................|
000005a0  03 c3 b1 04 08 04 19 04  2e 04 3c 04 50 04 5f 04  |..........<.P._.|
000005b0  74 04 c2 87 c3 a5 c2 95  c3 b7 34 05 c2 8b 77 15  |t.........4...w.|
000005c0  c3 9b c2 9f 05 3b c2 8b  05 c3 9b c2 8b 15 3b 77  |.....;........;w|
000005d0  05 c2 8b c3 87 15 c3 9b  4f 05 c2 8b c3 87 15 0e  |........O.......|
000005e0  c3 a5 c2 95 c2 9f 05 c2  8b c3 b7 48 15 c2 8b 27  |...........H...'|
000005f0  05 c2 a9 c2 8b 15 6d c3  af 15 0e c3 a5 0e c3 a5  |......m.........|
00000600  c3 a5 c2 9f 05 c2 8b c3  b7 48 15 c2 8b 27 05 6d  |.........H...'.m|
00000610  c2 8b 15 c2 a9 c3 af 15  0e c3 a5 c3 a5 c3 af 05  |................|
00000620  6d c2 8b 15 c2 a9 c3 af  15 3b 59 05 c3 91 c2 8b  |m........;Y.....|
00000630  15 0e c3 a5 c3 a5 c3 af  05 6d c2 8b 15 59 c3 af  |.........m...Y..|
00000640  15 c2 8b 59 05 c3 91 c2  8b 15 0e c3 a5 c3 a5 c3  |...Y............|
00000650  af 05 6d c2 8b 15 c2 a9  c3 af 15 c2 8b 59 05 63  |..m..........Y.c|
00000660  c2 8b 15 c2 b3 c2 8b 05  c2 8b c3 b7 16 15 0e c3  |................|
00000670  a5 c2 95 c2 9f 05 c2 8b  c3 b7 48 15 c2 8b 59 05  |..........H...Y.|
00000680  c3 9b c2 8b 15 c2 8b 59  05 c2 8b c3 af 15 0e c3  |.......Y........|
00000690  a5 c2 95 c3 af 05 c3 9b  c2 8b 15 63 c2 8b 05 c2  |...........c....|
000006a0  8b c3 af 15 63 c2 8b 05  c3 9b c2 8b 15 0e c3 a5  |....c...........|
000006b0  c2 95 c3 af 05 c3 9b c2  8b 15 3b c3 af 05 c2 a9  |..........;.....|
000006c0  c2 8b 15 c2 a9 27 15 0e  c3 a5 c2 95 c2 9f 05 c2  |.....'..........|
000006d0  8b c3 b7 48 15 c2 8b 59  05 c2 b3 c2 8b 15 c2 a9  |...H...Y........|
000006e0  59 15 c2 95 c3 af 05 3b  59 15 0e c3 a5 c2 95 c3  |Y......;Y.......|
000006f0  af 05 c2 8b c3 91 15 c3  9b c2 a9 15 0e c3 a5 c2  |................|
00000700  95 c3 b7 0c 05 c2 8b c2  8b 05 c2 8b c3 9b 15 c2  |................|
00000710  b3 3b 05 c2 8b c3 9b 15  c2 b3 3b 05 c2 8b c3 b7  |.;........;.....|
00000720  34 15 0e c3 a5 c2 95 c3  b7 02 05 c2 8b c2 8b 05  |4...............|
00000730  c2 8b c3 a5 15 c2 95 27  05 c3 87 c3 af 15 c2 8b  |.......'........|
00000740  c2 8b 05 c2 95 27 15 0e  c3 a5 c2 95 c3 b7 20 05  |.....'........ .|
00000750  c3 9b c2 9f 15 3b 77 15  0e c3 a5 c2 95 c3 af 05  |.....;w.........|
00000760  c2 8b c3 b7 48 15 c2 8b  c3 bb 48 05 c2 a9 c2 8b  |....H.....H.....|
00000770  15 6d c3 af 15 0e c3 a5  c3 a5 c3 af 05 c2 8b c3  |.m..............|
00000780  b7 48 15 c2 8b c3 bb 48  05 6d c2 8b 15 c2 a9 c3  |.H.....H.m......|
00000790  af 15 0e c3 a5 c2 95 c3  af 05 c2 8b c3 af 15 c2  |................|
000007a0  8b 59 05 c2 b3 c2 8b 15  63 59 15 c3 9b c3 af 05  |.Y......cY......|
000007b0  3b 59 15 0e c3 a5 c3 a5  c3 af 05 54 c2 8b 15 c2  |;Y.........T....|
000007c0  a9 c2 bd 15 54 c2 bd 15  0e c3 a5 c2 95 c3 af 05  |....T...........|
000007d0  c3 9b c2 8b 15 63 c2 8b  05 c2 8b c3 af 15 0e c3  |.....c..........|
000007e0  a5 c2 95 c3 af 05 c2 8b  c3 87 15 c3 9b 4f 15 0e  |.............O..|
000007f0  c3 a5 c2 95 c3 af 05 c2  b3 c3 af 15 c2 b3 27 15  |..............'.|
00000800  0e c3 a5 c2 95 c3 af 05  c2 8b c3 9b 15 c2 8b c2  |................|
00000810  8b 05 c2 8b 3b 15 c2 b3  c3 9b 05 c2 8b 3b 15 c2  |....;........;..|
00000820  b3 c3 9b 05 c2 8b 3b 15  0e c3 a5 c2 95 c3 af 05  |......;.........|
00000830  c3 9b c3 af 15 c2 8b 27  05 3b c3 af 15 0e c3 a5  |.......'.;......|
00000840  c2 95 c3 af 05 c2 8b c3  87 15 c3 9b 4f 15 63 c3  |............O.c.|
00000850  af 05 c2 8b c3 9b 15 0e  c3 a5 c2 95 c3 af 05 c3  |................|
00000860  9b c2 8b 15 c2 8b c2 8b  05 3b c3 af 15 c2 8b c2  |.........;......|
00000870  8b 05 c3 9b c2 8b 15 0e  c3 a5 c2 95 c3 b7 5c 05  |..............\.|
00000880  c2 b3 c3 bb 48 15 c2 b3  c3 b7 48 15 4b 3b 05 c2  |....H.....H.K;..|
00000890  bd c2 8b 15 0e c3 a5 c2  95 c2 9f 05 c2 8b c3 b7  |................|
000008a0  48 15 c2 8b c3 bb 48 05  c2 a9 c2 8b 15 6d c3 9b  |H.....H......m..|
000008b0  15 c2 a9 c2 8b 05 6d c3  af 15 0e c3 a5 c3 a5 c2  |......m.........|
000008c0  9f 05 6d c2 8b 15 59 c3  b7 16 15 c3 9b c2 bd 15  |..m...Y.........|
000008d0  0e c3 a5 c2 95 c2 9f 05  c2 a9 c2 8b 15 c2 bd c2  |................|
000008e0  bd 05 59 c3 b7 16 05 c2  bd c3 bb 16 15 59 c3 b7  |..Y..........Y..|
000008f0  16 05 6d c2 8b 15 c2 8b  c3 bb 48 15 0e c3 a5 c3  |..m.......H.....|
00000900  a5 c2 9f 05 3b c2 8b 15  c2 8b c3 9b 15 c3 91 c2  |....;...........|
00000910  8b 15 45 c2 8b 05 c2 8b  c3 af 15 c3 9b c2 8b 15  |..E.............|
00000920  0e c3 a5 c3 a5 c2 9f 05  3b c2 8b 15 c2 8b c3 9b  |........;.......|
00000930  15 c3 91 c2 8b 15 45 c2  8b 05 c2 8b c3 af 15 0e  |......E.........|
00000940  c3 a5 c3 a5 c2 9f 05 6d  c2 8b 15 59 c3 b7 16 15  |.......m...Y....|
00000950  c3 9b c2 bd 15 c2 8b 3b  05 c2 8b c3 9b 15 c2 8b  |.......;........|
00000960  3b 05 63 c2 8b 15 0e c3  a5 c2 95 c2 9f 05 c2 8b  |;.c.............|
00000970  c3 b7 48 15 c3 9b c3 bb  48 05 c2 8b c3 b7 48 15  |..H.....H.....H.|
00000980  3b 27 05 c3 9b c2 8b 15  0e c3 a5 c2 bd c2 9f 05  |;'..............|
00000990  c2 8b c3 b7 48 15 63 c3  bb 48 05 c3 9b c2 8b 15  |....H.c..H......|
000009a0  3b c3 b7 48 05 c3 9b c2  8b 15 0e c3 a5 c2 95 c2  |;..H............|
000009b0  9f 05 c3 9b c2 8b 15 3b  c3 b7 48 05 c2 a9 c2 8b  |.......;..H.....|
000009c0  15 c2 a9 c3 bb 48 15 0e  c3 a5 c2 95 c2 9f 05 c2  |.....H..........|
000009d0  8b c3 b7 48 15 c2 8b 27  05 c2 a9 c2 8b 15 c2 b3  |...H...'........|
000009e0  3b 15 c2 95 c3 b7 48 05  c2 8b 59 15 0e c3 a5 c2  |;.....H...Y.....|
000009f0  95 c2 9f 05 c2 8b c3 b7  2a 15 c3 9b c2 a9 15 0e  |........*.......|
00000a00  c3 a5 c2 95 c3 b7 5c 05  c2 8b c3 bb 48 15 c2 b3  |......\.....H...|
00000a10  c3 9b 15 c2 b3 3b 15 c2  8b c3 b7 48 15 0e c3 a5  |.....;.....H....|
00000a20  c2 95 c3 b7 5c 05 c2 8b  c3 bb 48 15 c3 9b c3 b7  |....\.....H.....|
00000a30  48 15 c2 8b c3 bb 48 15  0e c3 a5 c2 95 c3 87 05  |H.....H.........|
00000a40  c3 9b c3 af 15 3b 27 15  0e c3 a5 c2 95 c2 9f 05  |.....;'.........|
00000a50  c2 8b c3 b7 48 15 c2 8b  c3 bb 48 05 c2 a9 c2 8b  |....H.....H.....|
00000a60  15 6d c3 af 15 0e c3 a5  c2 95 c3 87 05 c3 9b c3  |.m..............|
00000a70  af 15 3b 27 15 c2 bd c3  b7 02 05 c2 a9 c2 a9 15  |..;'............|
00000a80  0e c3 a5 c2 95 c2 9f 05  c2 8b c3 b7 48 15 c2 8b  |............H...|
00000a90  c3 bb 48 05 c2 a9 c2 8b  15 6d c3 9b 15 c3 9b c3  |..H......m......|
00000aa0  af 05 c2 8b 59 15 0e c3  a5 c3 a5 c2 9f 05 63 c2  |....Y.........c.|
00000ab0  8b 15 63 c3 b7 48 15 0e  c3 a5 c2 95 c2 9f 05 c3  |..c..H..........|
00000ac0  9b c2 8b 15 63 c2 8b 05  c2 8b c3 b7 48 15 0e c3  |....c.......H...|
00000ad0  a5 c2 95 c2 9f 05 c2 8b  c3 b7 20 15 c3 9b c3 bb  |.......... .....|
00000ae0  20 15 0e c3 a5 c2 95 c2  9f 05 c2 b3 c3 b7 48 15  | .............H.|
00000af0  c2 b3 c3 bb 48 15 0e c3  a5 c2 95 c2 9f 05 c2 8b  |....H...........|
00000b00  c3 b7 48 15 c2 b3 3b 15  c2 b3 c3 9b 15 c2 8b c3  |..H...;.........|
00000b10  bb 48 15 0e c3 a5 c2 95  c2 9f 05 c3 9b c3 b7 48  |.H.............H|
00000b20  15 c2 8b c3 bb 48 05 3b  c3 b7 48 15 0e c3 a5 c2  |.....H.;..H.....|
00000b30  95 c2 9f 05 c2 b3 c3 9b  15 c2 b3 3b 15 63 c3 9b  |...........;.c..|
00000b40  05 c2 8b c3 af 15 0e c3  a5 c2 95 c2 9f 05 c3 9b  |................|
00000b50  c2 8b 15 c2 8b c2 8b 05  3b c3 b7 48 15 c2 8b c2  |........;..H....|
00000b60  8b 05 c3 9b c2 8b 15 0e  c3 a5 c2 95 c3 87 05 c3  |................|
00000b70  9b c3 af 15 3b 27 15 c2  b3 c2 b3 05 c2 8b c2 9f  |....;'..........|
00000b80  15 0e c3 a5 c2 bd c2 9f  05 c2 8b c3 b7 48 15 77  |.............H.w|
00000b90  c3 bb 34 05 c2 9f 77 15  63 c3 b7 48 05 c3 9b c2  |..4...w.c..H....|
00000ba0  8b 15 0e c3 a5 c2 95 c3  87 05 c3 9b c2 9f 15 3b  |...............;|
00000bb0  c3 b7 0c 05 c3 9b c3 bb  0c 15 3b c3 b7 0c 05 c3  |..........;.....|
00000bc0  9b c2 8b 15 0e c3 a5 c2  95 c2 9f 05 c2 a9 c2 8b  |................|
00000bd0  15 c2 86 c3 9b 15 c2 90  c2 8b 05 6d c3 af 15 0e  |...........m....|
00000be0  c3 a5 c2 95 c2 9f 05 c3  9b c2 8b 15 c2 8b c2 8b  |................|
00000bf0  05 3b c3 b7 48 15 c2 8b  c2 8b 05 c3 9b c2 8b 15  |.;..H...........|
00000c00  0e c3 a5 c3 a5 c2 9f 05  3b c2 8b 15 c2 8b c3 9b  |........;.......|
00000c10  15 c2 8b c3 af 15 0e c3  a5 c2 95 c3 b7 20 05 c3  |............. ..|
00000c20  9b c2 9f 15 3b 77 15 c2  b3 c3 bb 0c 05 63 c3 b7  |....;w.......c..|
00000c30  02 15 0e c3 a5 c2 95 c2  9f 05 c3 9b c2 8b 15 c2  |................|
00000c40  8b c2 8b 05 54 c3 b7 48  15 0e c3 a5 c2 95 c3 b7  |....T..H........|
00000c50  20 05 c3 9b c2 9f 15 3b  77 15 c2 95 31 05 c3 87  | ......;w...1...|
00000c60  c2 9f 15 4f 77 15 0e c3  a5 c2 95 c3 87 05 c3 9b  |...Ow...........|
00000c70  c2 9f 15 3b 77 15 c2 b3  c3 b7 20 05 c2 b3 c3 bb  |...;w..... .....|
00000c80  02 15 0e 00 00 00 5a 00  00 00 5a 00 00 00 5a 00  |......Z...Z...Z.|
00000c90  00 00 5a 00 00 00 5a 00  00 00 5a 00 00 00 5a 00  |..Z...Z...Z...Z.|
*
00000d70  00 00 5a 00 00 00 5a 00  00 00 5a 00 00           |..Z...Z...Z..|
00000d7d

And here is what my font.html file looks like to test it out:

<style>
@font-face {
  font-family: 'FooLatin';
  src: url('font.ttf') format('truetype');
}

body {
  font-family: 'FooLatin';
}
</style>

<body>
  Hello World
</body>

I am not sure if I am using truetype or opentype. But with this configuration, I get this error in the browser:

font.html:1 Failed to decode downloaded font: file:~/tests/font.ttf
font.html:1 OTS parsing error: hmtx: invalid table offset

If I change the @font-face definition to use format('embedded-opentype'):

<style>
@font-face {
  font-family: 'FooLatin';
  src: url('font.ttf') format('embedded-opentype');
}

body {
  font-family: 'FooLatin';
}
</style>

<body>
  Hello World
</body>

..the error goes away, but nothing happens, it still doesn't work.

I just noticed this is actually a semi-helpful error:

hmtx: invalid table offset

I will look more into that. But I wanted to also ask if there are any techniques to debug fonts like in this situation. I have pasted above the hexdump of the program, but I don't know what to look for.

Solution

The hex dump shows a number of serious problems:

The Table Directory entries are not aligned properly. Each tag/checkSum/offset/length entry should occupy 16 bytes. From the dump, it looks like your entries have some extra bytes
The Table Directory entries are not sorted in increasing order by tag
The offsets in the Table Directory are suspicious (offset for hmtx is 8, which is before the Table Directory itself). The offsets in the Table Directory are absolute (from byte 0 of the font), not relative (although even a relative offset of 8 would be wrong for that case).
The correct tag for the CFF table is 'CFF' (i.e. 'C', 'F', 'F', ' ', and should be uppercase, not lowercase).
The Table Directory does not appear to have a name or OS/2 table. These are both required tables (see the OpenType specification). While you might be able to get away with no OS/2 table on some platforms, the name table really is needed for a minimal valid font.
This is actually minor, but CFF-based fonts should use the extension .otf, not .ttf. But you should really solve the items I mention above first, and ensure that the table directory offset & length entries correctly point to the right data within the rest of the file.

In general I would recommend spending some time, perhaps a significant amount of time, studying the OpenType specification.