I am aware that any Unicode character can be inserted into an HTML document via the following format:
�
...where 0000 is the character code of the desired character
My question is: which of these characters has the most widespread availability when it comes to the client's browser being able to display the character?
In other words, what are the ranges of codes that should be used in an HTML document that is going to be widely deployed?
Whether you enter them via &#xNNNN; character reference or just by typing or copy-pasting them directly into the page, it's purely a question of fonts. If the user has a glyph for the character in the fonts their web browser knows about, it'll work. If they don't, it'll be a little box.
fileformat.info have font-availability details that can be of help for guessing how widespread glyph support is. eg. hit the Fonts link on this page for U+25B6. For me, this renders on most platforms but not on a vanilla XP install with IE6 (IE7 picks it up from Lucida Sans Unicode which IE6 FSR doesn't bother look at).
◄ ► U+25C4, U+25BA Left/Right-Pointing Pointer glyphs seem to be more widely available than
◀ ▶ U+25C0, U+25B6 Left/Right-Pointing Triangle glyphs
so I'd use those instead. In particlar they render in XP+IE6 with default fonts because they are present in Arial. Which renders them more like Triangle than Pointer is supposed to be. (sigh) OTOH its up/down Triangles are fine.
SO uses ► for list bullets in the ‘how to format’ box over there. ⇗
[response to comment in edit instead of comment as site JS appears to be playing up]
There's also ‣, U+2023 Triangular Bullet, which is in a block of widely-available typographical-niceties. Unfortunately, again, it's not in Arial or other fonts IE6 on a default XP will use. Neither is ▸ U+25B8 Black Right-Pointing Small Triangle.
French-style quotes (guillemets) ‹ › « » will be available absolutely everywhere, but using them for arrows might be considered a bit of an abuse.