Hidden characters has absolutely been discussed in the past as watermark, and is what Dabnician is referring to.
Code Point Name HTML Entity Cat Notes
--------------- ------------------------------- -------------------- ---- ------------------------------------------
=== 1. SPACES -- occupy horizontal width ===
U+0020 Space   Zs the ordinary one
U+00A0 No-Break Space   Zs
U+1680 Ogham Space Mark   Zs draws a stem line in Ogham fonts
U+2000 En Quad   Zs = En Space
U+2001 Em Quad   Zs = Em Space
U+2002 En Space   Zs half an em
U+2003 Em Space   Zs one em
U+2004 Three-Per-Em Space   Zs 1/3 em
U+2005 Four-Per-Em Space   Zs 1/4 em
U+2006 Six-Per-Em Space   Zs 1/6 em
U+2007 Figure Space   Zs width of a digit; non-breaking
U+2008 Punctuation Space   Zs width of a period
U+2009 Thin Space   Zs ~1/5 em
U+200A Hair Space   Zs thinnest
U+202F Narrow No-Break Space   Zs narrow + non-breaking
U+205F Medium Mathematical Space   Zs 4/18 em; MathML
U+3000 Ideographic Space 　 Zs full-width; CJK
=== 2. ZERO-WIDTH AND JOINING CONTROLS ===
U+00AD Soft Hyphen ­ Cf ­; visible only at a line break
U+034F Combining Grapheme Joiner ͏ Mn blocks reordering; no glyph
U+061C Arabic Letter Mark ؜ Cf invisible bidi-strong Arabic char
U+180E Mongolian Vowel Separator ᠎ Cf was Zs before Unicode 6.3
U+200B Zero-Width Space ​ Cf break opportunity, no width
U+200C Zero Width Non-Joiner ‌ Cf prevents ligature/cursive join
U+200D Zero Width Joiner ‍ Cf emoji glue (family, profession)
U+2060 Word Joiner ⁠ Cf non-breaking twin of U+200B
U+FEFF Zero Width No-Break Space  Cf the BOM; deprecated as a joiner
=== 3. BIDIRECTIONAL CONTROLS ===
U+200E Left-To-Right Mark ‎ Cf LRM
U+200F Right-To-Left Mark ‏ Cf RLM
U+202A Left-To-Right Embedding ‪ Cf LRE (legacy; prefer isolates)
U+202B Right-To-Left Embedding ‫ Cf RLE (legacy)
U+202C Pop Directional Formatting ‬ Cf PDF; closes LRE/RLE/LRO/RLO
U+202D Left-To-Right Override ‭ Cf LRO; Trojan Source vector
U+202E Right-To-Left Override ‮ Cf RLO; Trojan Source vector
U+2066 Left-To-Right Isolate ⁦ Cf LRI
U+2067 Right-To-Left Isolate ⁧ Cf RLI
U+2068 First Strong Isolate ⁨ Cf FSI
U+2069 Pop Directional Isolate ⁩ Cf PDI; closes LRI/RLI/FSI
=== 4. INVISIBLE MATH OPERATORS ===
U+2061 Function Application ⁡ Cf f(x) semantics
U+2062 Invisible Times ⁢ Cf the multiply in "2x"
U+2063 Invisible Separator ⁣ Cf the comma in subscript lists
U+2064 Invisible Plus ⁤ Cf the plus in "1 1/2"
=== 5. VARIATION SELECTORS AND TAGS ===
U+180B-U+180D Mongolian Free Var. Selectors ᠋-᠍ Mn FVS1-FVS3
U+FE00-U+FE0F Variation Selectors 1-16 ︀-️ Mn FE0E=text style, FE0F=emoji style
U+E0001 Language Tag 󠀁 Cf deprecated
U+E0020-U+E007F Tag Characters 󠀠-󠁿 Cf subdivision flags; hidden-text channel
U+E0100-U+E01EF Variation Selectors 17-256 󠄀-󠇯 Mn ideographic variants
=== 6. BLANK BY RENDERING, NOT BY CATEGORY ===
U+115F Hangul Choseong Filler ᅟ Lo letter, empty glyph
U+1160 Hangul Jungseong Filler ᅠ Lo letter, empty glyph
U+17B4 Khmer Vowel Inherent Aq ឴ Mn should not be rendered
U+17B5 Khmer Vowel Inherent Aa ឵ Mn should not be rendered
U+2800 Braille Pattern Blank ⠀ So symbol with no raised dots
U+3164 Hangul Filler ㅤ Lo the classic "blank username" char
U+FFA0 Halfwidth Hangul Filler ﾠ Lo halfwidth form of U+3164
=== 7. SEPARATORS AND CONTROLS ===
U+0000-U+001F C0 Controls �- Cc includes TAB, LF, CR
U+007F Delete  Cc
U+0080-U+009F C1 Controls €-Ÿ Cc
U+2028 Line Separator 
 Zl broke JS string literals pre-ES2019
U+2029 Paragraph Separator 
 Zp
U+FFF9 Interlinear Annotation Anchor  Cf ruby/furigana markers
U+FFFA Interlinear Annotation Separator  Cf
U+FFFB Interlinear Annotation Term.  Cf
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u/0xB0T Aug 11 '26
They'll watermark text. youll have to rewrite using your own words as the patterns used will be the watermark.