currently the definition is as in https://htmlpreview.github.io/?https://github.com/jgm/djot/blob/master/doc/syntax.html#symbols

which says:

Surrounding a word with : signs creates a “symbol,” which by default is just rendered literally ...

so the implementation is highly parser and renderer specific.

• in the Haskell implementation at https://github.com/jgm/djoths/blob/f1ab25c2205f46ab4923ed2a66ed91b4971eda04/src/Djot/Djot.hs#L400C11-L400C65 a word is defined as a literal.
• but here https://github.com/jgm/djoths/blob/f1ab25c2205f46ab4923ed2a66ed91b4971eda04/src/Djot/Inlines.hs#L214 it is:

pSymbol = do
  asciiChar ':'
  bs <- byteStringOf $ skipSome (skipSatisfyByte
                                    (\c -> c == '+' || c == '-' ||
                                         (isAscii c && isAlphaNum c)))
  asciiChar ':'

• in the Lua implemantaion at https://github.com/jgm/djot.lua/blob/a0583ef8270d025b3e86ef521b35f7397cf7215b/djot/inline.lua#L403 a word is defined as

-- 58 = :
    [58] = function(self, pos, endpos)
      local sp, ep = bounded_find(self.subject, "^%:[%w_+-]+%:", pos, endpos)
      if sp then
        self:add_match(sp, ep, "symbol")
        return ep + 1
      else
        self:add_match(pos, pos, "str")
        return pos + 1
      end
    end,

so it seems that as least comparing the Haskell and Lua implementations there is some disagreement

Proposal

• allow the syntax of html entities/symbol/emojies -- https://www.w3schools.com/html/html_entities.asp
• allow the syntax of adobe postscript-style glyph names -- https://github.com/adobe-type-tools/agl-specification
• allow the syntax of opentype font glyph names -- https://silnrsi.github.io/FDBP/en-US/Glyph_Naming.html
• allow common naming practices by common css icon-libraries such as Font Awesome Icons, Google Material Design Icons, or Bootstrap Icons (ie.[a-zA-Z0-9\_\-]+).

Use-Cases

XML/HTML Renderer

HTML Entity

• :apos: to be rendered as '
• :Euro: to be rendered as &Euro;

HTML Entity Code Point

• :#60: to be rendered as < or <
• :#x2014: to be rendered as — or — or — or "—"

Icon Font Glyph Name

• :fa-bars: to be rendered as <i class="fa fa-bars"></i>
• :fa+fa-bars: to be rendered as <i class="fa fa-bars"></i>

This may be subject to that actual implementation and/or configuration of the html-renderer backend.

Icon or Symbol Font Glyph Name for PDF, Image, or Unicode Text Renderer

• :a19: to be rendered as "✓" (from Zapf Dingbats font) -- (U+2713 CHECK MARK ✓, ✓)

Possible Variation

it might be desirable to clearly separate the verbatim html entities from glyph names by using a prefix for indication.

• :*a19: to be rendered as "✓" (from Zapf Dingbats font) -- (U+2713 CHECK MARK ✓, ✓)

Possible candidates would be : ^, !, &, $, %, /, =, ?, +, ~, *, #

So a possible syntax could be in perl-style regular expression:

/^[\^\!\&\$\%\/\=\?\+\~\*\#]?[[:alnum:]\.\_\-\+]+$/

Graceful Fallback Mechanism

where a renderer backend might not be able to recognize which symbol or glyph to actually render it might fallback i the following ways:

pure text style

• :apos: to be rendered as :apos:
• :Euro: to be rendered as :Euro:
• :#60: to be rendered as :#60:
• :#x2014: to be rendered as :#x2014:
• :fa-bars: to be rendered as :fa-bars:
• :fa+fa-bars: to be rendered as :fa+fa-bars:
• :a19: to be rendered as :a19:
• :*a19: to be rendered as :*a19:

html style

• :fa-bars: to be rendered as <code>:fa-bars:</code>
• :fa+fa-bars: to be rendered as <code>:fa+fa-bars:</code>
• :a19: to be rendered as <code>:a19:</code>
• :*a19: to be rendered as <code>:*a19:</code>

Entities that should always be recognized

• Entities as specified as numeric eg.  , , etc.
• https://en.wikipedia.org/wiki/List_of_XML_and_HTML_character_entity_references