Is there a simple way of creating vowels with horizontal bars on top to
represent long vowels? And is there a way of making it appear on web
This is a macron, and the standard (ISO) symbols are (eg) &emacron; for
an e with an overbar, but the difficulty I face is I need the macron on
consonants, so I've had to define (eg) &mmacron; as a local standard.
Representing it in an SGML instance is unconnected with how it appears
in print. If you use something manipulable like TeX, it's trivial (you
convert it to \=e) but if you want it in a Web browser, that's
something you'll have to argue with your browser maker: most of them
are wholly uninterested in supporting anything outside ISO latin-1.
If you use a browser which lets you specify font and size, you could
find or cook up a font for local display, but no-one else will have
it, and <font> isn't HTML anyway, yet.
This is the most frequently used symbol in print, but I've never seen it
in a computer environment. The circumflex is a possible substitute, but
it seems difficult or impossible to put in some vowels, e.g. 'y.'
TeX has been able to do this trivially for over 15 years.