Below are several zero-width Unicode characters, placed between underscores. Can your browser display them? Your text editor? Your terminal? To understand why not being able to display them might be a problem, read:
As a point of reference, here are a few positive-width Unicode characters:
0020: _ _ | 00E9: _é_ | 03A9: _Ω_ | 5B57: _字_ | 1F407: _🐇_
0016: __ synchronous idle 034F: _͏_ combining grapheme joiner 061C: __ Arabic letter mark 115F: _ᅟ_ Hangul choseong filler 1160: _ᅠ_ Hangul jungseong filler 17B4: _឴_ Khmer vowel inherent aq 17B5: _឵_ Khmer vowel inherent aa 180E: __ Mongolian vowel separator 200B: __ zero width space 200C: __ zero width non-joiner 200D: __ zero width joiner 200E: __ left-to-right mark 200F: __ right-to-left mark 2028: _ _ line separator 2029: _ _ paragraph separator 202A: __ left-to-right embedding 202B: __ right-to-left embedding 202C: __ pop directional formatting 202D: __ left-to-right override 202E: __ right-to-left override 2060: __ word joiner 2061: __ function application 2062: __ invisible times 2063: __ invisible separator 2064: __ invisible plus 2066: __ left-to-right isolate 2067: __ right-to-left isolate 2068: __ first strong isolate 2069: __ pop directional isolate 206A: __ inhibit symmetric swapping (deprecated) 206B: __ activate symmetric swapping (deprecated) 206C: __ inhibit arabic form shaping (deprecated) 206D: __ activate arabic form shaping (deprecated) 206E: __ national digit shapes (deprecated) 206F: __ nominal digit shapes (deprecated) 3164: _ㅤ_ Hangul filler FEFF: __ zero width no-break space FFA0: _ᅠ_ halfwidth Hangul filler FFF9: __ interlinear annotation anchor FFFA: __ interlinear annotation separator FFFB: __ interlinear annotation terminator 1D159: _𝅙_ musical symbol null notehead 1D173: __ musical symbol begin beam 1D174: __ musical symbol end beam 1D175: __ musical symbol begin tie 1D176: __ musical symbol end tie 1D177: __ musical symbol begin slur 1D178: __ musical symbol end slur 1D179: __ musical symbol begin phrase 1D17A: __ musical symbol end phrase E0000: __ unassigned E0001: __ language tag (deprecated) E0002: __ unassigned ... (E0002-E0019 unassigned) E0019: __ unassigned E0020: __ ... (E0020-E007F formerly used for tagging texts by language) E007F: __
Contact me if you know of any other characters that should be listed here.
This test page is also available on the Web and on Gopher:
https://nervuri.net/zero-width
gopher://rawtext.club/0/~nervuri/zero-width.txt
The web version also contains links to programs which pass the test (more or less).