[aricb] HTML character entity references are defined as part of the HTML specification: http://www.w3.org/TR/REC-html40/sgml/entities.html

The following code defines two commands, htmlentities::insertentities and htmlentities::removeentities.  The first command replaces Unicode characters with HTML character entity references; the second command replaces HTML character entity references with the literal Unicode characters they represent.  There's nothing fancy about the code, but hopefully it's useful to somebody.

  namespace eval htmlentities {
      variable map [list \
          \u00a0 &nbsp\;    \
          \u00a1 &iexcl\;   \
          \u00a2 &cent\;    \
          \u00a3 &pound\;   \
          \u00a4 &curren\;  \
          \u00a5 &yen\;     \
          \u00a6 &brvbar\;  \
          \u00a7 &sect\;    \
          \u00a8 &uml\;     \
          \u00a9 &copy\;    \
          \u00aa &ordf\;    \
          \u00ab &laquo\;   \
          \u00ac &not\;     \
          \u00ad &shy\;     \
          \u00ae &reg\;     \
          \u00af &macr\;    \
          \u00b0 &deg\;     \
          \u00b1 &plusmn\;  \
          \u00b2 &sup2\;    \
          \u00b3 &sup3\;    \
          \u00b4 &acute\;   \
          \u00b5 &micro\;   \
          \u00b6 &para\;    \
          \u00b7 &middot\;  \
          \u00b8 &cedil\;   \
          \u00b9 &sup1\;    \
          \u00ba &ordm\;    \
          \u00bb &raquo\;   \
          \u00bc &frac14\;  \
          \u00bd &frac12\;  \
          \u00be &frac34\;  \
          \u00bf &iquest\;  \
          \u00c0 &Agrave\;  \
          \u00c1 &Aacute\;  \
          \u00c2 &Acirc\;   \
          \u00c3 &Atilde\;  \
          \u00c4 &Auml\;    \
          \u00c5 &Aring\;   \
          \u00c6 &AElig\;   \
          \u00c7 &Ccedil\;  \
          \u00c8 &Egrave\;  \
          \u00c9 &Eacute\;  \
          \u00ca &Ecirc\;   \
          \u00cb &Euml\;    \
          \u00cc &Igrave\;  \
          \u00cd &Iacute\;  \
          \u00ce &Icirc\;   \
          \u00cf &Iuml\;    \
          \u00d0 &ETH\;     \
          \u00d1 &Ntilde\;  \
          \u00d2 &Ograve\;  \
          \u00d3 &Oacute\;  \
          \u00d4 &Ocirc\;   \
          \u00d5 &Otilde\;  \
          \u00d6 &Ouml\;    \
          \u00d7 &times\;   \
          \u00d8 &Oslash\;  \
          \u00d9 &Ugrave\;  \
          \u00da &Uacute\;  \
          \u00db &Ucirc\;   \
          \u00dc &Uuml\;    \
          \u00dd &Yacute\;  \
          \u00de &THORN\;   \
          \u00df &szlig\;   \
          \u00e0 &agrave\;  \
          \u00e1 &aacute\;  \
          \u00e2 &acirc\;   \
          \u00e3 &atilde\;  \
          \u00e4 &auml\;    \
          \u00e5 &aring\;   \
          \u00e6 &aelig\;   \
          \u00e7 &ccedil\;  \
          \u00e8 &egrave\;  \
          \u00e9 &eacute\;  \
          \u00ea &ecirc\;   \
          \u00eb &euml\;    \
          \u00ec &igrave\;  \
          \u00ed &iacute\;  \
          \u00ee &icirc\;   \
          \u00ef &iuml\;    \
          \u00f0 &eth\;     \
          \u00f1 &ntilde\;  \
          \u00f2 &ograve\;  \
          \u00f3 &oacute\;  \
          \u00f4 &ocirc\;   \
          \u00f5 &otilde\;  \
          \u00f6 &ouml\;    \
          \u00f7 &divide\;  \
          \u00f8 &oslash\;  \
          \u00f9 &ugrave\;  \
          \u00fa &uacute\;  \
          \u00fb &ucirc\;   \
          \u00fc &uuml\;    \
          \u00fd &yacute\;  \
          \u00fe &thorn\;   \
          \u00ff &yuml\;    \
          \u0192 &fnof\;    \
          \u0391 &Alpha\;   \
          \u0392 &Beta\;    \
          \u0393 &Gamma\;   \
          \u0394 &Delta\;   \
          \u0395 &Epsilon\; \
          \u0396 &Zeta\;    \
          \u0397 &Eta\;     \
          \u0398 &Theta\;   \
          \u0399 &Iota\;    \
          \u039a &Kappa\;   \
          \u039b &Lambda\;  \
          \u039c &Mu\;      \
          \u039d &Nu\;      \
          \u039e &Xi\;      \
          \u039f &Omicron\; \
          \u03a0 &Pi\;      \
          \u03a1 &Rho\;     \
          \u03a3 &Sigma\;   \
          \u03a4 &Tau\;     \
          \u03a5 &Upsilon\; \
          \u03a6 &Phi\;     \
          \u03a7 &Chi\;     \
          \u03a8 &Psi\;     \
          \u03a9 &Omega\;   \
          \u03b1 &alpha\;   \
          \u03b2 &beta\;    \
          \u03b3 &gamma\;   \
          \u03b4 &delta\;   \
          \u03b5 &epsilon\; \
          \u03b6 &zeta\;    \
          \u03b7 &eta\;     \
          \u03b8 &theta\;   \
          \u03b9 &iota\;    \
          \u03ba &kappa\;   \
          \u03bb &lambda\;  \
          \u03bc &mu\;      \
          \u03bd &nu\;      \
          \u03be &xi\;      \
          \u03bf &omicron\; \
          \u03c0 &pi\;      \
          \u03c1 &rho\;     \
          \u03c2 &sigmaf\;  \
          \u03c3 &sigma\;   \
          \u03c4 &tau\;     \
          \u03c5 &upsilon\; \
          \u03c6 &phi\;     \
          \u03c7 &chi\;     \
          \u03c8 &psi\;     \
          \u03c9 &omega\;   \
          \u03d1 &thetasym\; \
          \u03d2 &upsih\;   \
          \u03d6 &piv\;     \
          \u2022 &bull\;    \
          \u2026 &hellip\;  \
          \u2032 &prime\;   \
          \u2033 &Prime\;   \
          \u203e &oline\;   \
          \u2044 &frasl\;   \
          \u2118 &weierp\;  \
          \u2111 &image\;   \
          \u211c &real\;    \
          \u2122 &trade\;   \
          \u2135 &alefsym\; \
          \u2190 &larr\;    \
          \u2191 &uarr\;    \
          \u2192 &rarr\;    \
          \u2193 &darr\;    \
          \u2194 &harr\;    \
          \u21b5 &crarr\;   \
          \u21d0 &lArr\;    \
          \u21d1 &uArr\;    \
          \u21d2 &rArr\;    \
          \u21d3 &dArr\;    \
          \u21d4 &hArr\;    \
          \u2200 &forall\;  \
          \u2202 &part\;    \
          \u2203 &exist\;   \
          \u2205 &empty\;   \
          \u2207 &nabla\;   \
          \u2208 &isin\;    \
          \u2209 &notin\;   \
          \u220b &ni\;      \
          \u220f &prod\;    \
          \u2211 &sum\;     \
          \u2212 &minus\;   \
          \u2217 &lowast\;  \
          \u221a &radic\;   \
          \u221d &prop\;    \
          \u221e &infin\;   \
          \u2220 &ang\;     \
          \u2227 &and\;     \
          \u2228 &or\;      \
          \u2229 &cap\;     \
          \u222a &cup\;     \
          \u222b &int\;     \
          \u2234 &there4\;  \
          \u223c &sim\;     \
          \u2245 &cong\;    \
          \u2248 &asymp\;   \
          \u2260 &ne\;      \
          \u2261 &equiv\;   \
          \u2264 &le\;      \
          \u2265 &ge\;      \
          \u2282 &sub\;     \
          \u2283 &sup\;     \
          \u2284 &nsub\;    \
          \u2286 &sube\;    \
          \u2287 &supe\;    \
          \u2295 &oplus\;   \
          \u2297 &otimes\;  \
          \u22a5 &perp\;    \
          \u22c5 &sdot\;    \
          \u2308 &lceil\;   \
          \u2309 &rceil\;   \
          \u230a &lfloor\;  \
          \u230b &rfloor\;  \
          \u2329 &lang\;    \
          \u232a &rang\;    \
          \u25ca &loz\;     \
          \u2660 &spades\;  \
          \u2663 &clubs\;   \
          \u2665 &hearts\;  \
          \u2666 &diams\;   \
          \u0022 &quot\;    \
          \u0026 &amp\;     \
          \u003c &lt\;      \
          \u003e &gt\;      \
          \u0152 &OElig\;   \
          \u0153 &oelig\;   \
          \u0160 &Scaron\;  \
          \u0161 &scaron\;  \
          \u0178 &Yuml\;    \
          \u02c6 &circ\;    \
          \u02dc &tilde\;   \
          \u2002 &ensp\;    \
          \u2003 &emsp\;    \
          \u2009 &thinsp\;  \
          \u200c &zwnj\;    \
          \u200d &zwj\;     \
          \u200e &lrm\;     \
          \u200f &rlm\;     \
          \u2013 &ndash\;   \
          \u2014 &mdash\;   \
          \u2018 &lsquo\;   \
          \u2019 &rsquo\;   \
          \u201a &sbquo\;   \
          \u201c &ldquo\;   \
          \u201d &rdquo\;   \
          \u201e &bdquo\;   \
          \u2020 &dagger\;  \
          \u2021 &Dagger\;  \
          \u2030 &permil\;  \
          \u2039 &lsaquo\;  \
          \u203a &rsaquo\;  \
          \u20ac &euro\;    ]
  
      set reversemap [lreverse $map]
      
      proc insertentities {string} {
          variable map
          return [string map $map $string]
      }
  
      proc removeentities {string} {
          variable reversemap
          return [string map $reversemap $string]
      }
  }

----
[AMG]: Here's the same code, written in a more compressed manner.  Be careful that you use [[[source] -encoding utf-8 $filename]] to load this file into your interpreter.  I took the liberty of using [namespace ensemble]s and renaming the commands.

======
namespace eval htmlentities {
    namespace ensemble create -subcommands {encode decode}
    set encode [list {*}{
          &nbsp\; ¡ &iexcl\; ¢ &cent\; £ &pound\; ¤ &curren\; ¥ &yen\; ¦
        &brvbar\; § &sect\; ¨ &uml\; © &copy\; ª &ordf\; « &laquo\; ¬ &not\; ­
        &shy\; ® &reg\; ¯ &macr\; ° &deg\; ± &plusmn\; ² &sup2\; ³ &sup3\; ´
        &acute\; µ &micro\; ¶ &para\; · &middot\; ¸ &cedil\; ¹ &sup1\; º
        &ordm\; » &raquo\; ¼ &frac14\; ½ &frac12\; ¾ &frac34\; ¿ &iquest\; À
        &Agrave\; Á &Aacute\; Â &Acirc\; Ã &Atilde\; Ä &Auml\; Å &Aring\; Æ
        &AElig\; Ç &Ccedil\; È &Egrave\; É &Eacute\; Ê &Ecirc\; Ë &Euml\; Ì
        &Igrave\; Í &Iacute\; Î &Icirc\; Ï &Iuml\; Ð &ETH\; Ñ &Ntilde\; Ò
        &Ograve\; Ó &Oacute\; Ô &Ocirc\; Õ &Otilde\; Ö &Ouml\; × &times\; Ø
        &Oslash\; Ù &Ugrave\; Ú &Uacute\; Û &Ucirc\; Ü &Uuml\; Ý &Yacute\; Þ
        &THORN\; ß &szlig\; à &agrave\; á &aacute\; â &acirc\; ã &atilde\; ä
        &auml\; å &aring\; æ &aelig\; ç &ccedil\; è &egrave\; é &eacute\; ê
        &ecirc\; ë &euml\; ì &igrave\; í &iacute\; î &icirc\; ï &iuml\; ð
        &eth\; ñ &ntilde\; ò &ograve\; ó &oacute\; ô &ocirc\; õ &otilde\; ö
        &ouml\; ÷ &divide\; ø &oslash\; ù &ugrave\; ú &uacute\; û &ucirc\; ü
        &uuml\; ý &yacute\; þ &thorn\; ÿ &yuml\; ƒ &fnof\; Α &Alpha\; Β &Beta\;
        Γ &Gamma\; Δ &Delta\; Ε &Epsilon\; Ζ &Zeta\; Η &Eta\; Θ &Theta\; Ι
        &Iota\; Κ &Kappa\; Λ &Lambda\; Μ &Mu\; Ν &Nu\; Ξ &Xi\; Ο &Omicron\; Π
        &Pi\; Ρ &Rho\; Σ &Sigma\; Τ &Tau\; Υ &Upsilon\; Φ &Phi\; Χ &Chi\; Ψ
        &Psi\; Ω &Omega\; α &alpha\; β &beta\; γ &gamma\; δ &delta\; ε
        &epsilon\; ζ &zeta\; η &eta\; θ &theta\; ι &iota\; κ &kappa\; λ
        &lambda\; μ &mu\; ν &nu\; ξ &xi\; ο &omicron\; π &pi\; ρ &rho\; ς
        &sigmaf\; σ &sigma\; τ &tau\; υ &upsilon\; φ &phi\; χ &chi\; ψ &psi\; ω
        &omega\; ϑ &thetasym\; ϒ &upsih\; ϖ &piv\; • &bull\; … &hellip\; ′
        &prime\; ″ &Prime\; ‾ &oline\; ⁄ &frasl\; ℘ &weierp\; ℑ &image\; ℜ
        &real\; ™ &trade\; ℵ &alefsym\; ← &larr\; ↑ &uarr\; → &rarr\; ↓ &darr\;
        ↔ &harr\; ↵ &crarr\; ⇐ &lArr\; ⇑ &uArr\; ⇒ &rArr\; ⇓ &dArr\; ⇔ &hArr\;
        ∀ &forall\; ∂ &part\; ∃ &exist\; ∅ &empty\; ∇ &nabla\; ∈ &isin\; ∉
        &notin\; ∋ &ni\; ∏ &prod\; ∑ &sum\; − &minus\; ∗ &lowast\; √ &radic\; ∝
        &prop\; ∞ &infin\; ∠ &ang\; ∧ &and\; ∨ &or\; ∩ &cap\; ∪ &cup\; ∫ &int\;
        ∴ &there4\; ∼ &sim\; ≅ &cong\; ≈ &asymp\; ≠ &ne\; ≡ &equiv\; ≤ &le\; ≥
        &ge\; ⊂ &sub\; ⊃ &sup\; ⊄ &nsub\; ⊆ &sube\; ⊇ &supe\; ⊕ &oplus\; ⊗
        &otimes\; ⊥ &perp\; ⋅ &sdot\; ⌈ &lceil\; ⌉ &rceil\; ⌊ &lfloor\; ⌋
        &rfloor\; 〈 &lang\; 〉 &rang\; ◊ &loz\; ♠ &spades\; ♣ &clubs\; ♥
        &hearts\; ♦ &diams\; \" &quot\; & &amp\; < &lt\; > &gt\; Œ &OElig\; œ
        &oelig\; Š &Scaron\; š &scaron\; Ÿ &Yuml\; ˆ &circ\; ˜ &tilde\;  
        &ensp\;   &emsp\;   &thinsp\; ‌ &zwnj\; ‍ &zwj\; ‎
        &lrm\; ‏ &rlm\; – &ndash\; — &mdash\; ‘ &lsquo\; ’ &rsquo\; ‚
        &sbquo\; “ &ldquo\; ” &rdquo\; „ &bdquo\; † &dagger\; ‡ &Dagger\; ‰
        &permil\; ‹ &lsaquo\; › &rsaquo\; € &euro\;
    }]
    set decode [lreverse $encode]
    proc encode {string} {
        variable encode
        string map $encode $string
    }
    proc decode {string} {
        variable decode
        string map $decode $string
    }
}
======

By the way, that's a very interesting use for [[[lreverse]]]!
  
<<categories>> HTML