Unicode, which is a consortium founded in 1991, aims to provide a universal standards to allow each glyphs in whatever scripting to have a unique code and is close to release version 5.0.

Use

Fundamental Web use

For the web, a full unicode fonts, implies that it will be possible to display all web pages in whatever languages to reach informations. For example, Wikipedia make a heavy use of Unicode.

Math use

For math, in the web, Unicode is just mandatory ! A lot of informations use specific characters (more than 2000 are available in Unicode), so they need to be properly displayed. Useful requirements on this could be found here.

Attempt

StixFonts

Stixfonts is an old project (since 1995) which has tried to give to all a set of fonts that serve the scientific and engineering community. They are lacking of 152 glyphs to complete their requirements.

DejaVu

DejaVu is a recent project (since 2004), hosted by Sourceforge, which is now giving good results and is free as in speech and as in beer. Its coverage is, for the moment, not high but grow up fast at each version (current is 2.5).

Others

And what about Arial, Unimath or Lucida ?

These fonts don't cover a high part of recent Unicode version and are not so widely accessible due to license requirement.

May be we have to wait for a Wikifont ?