A (quite large) subset of Unicode characters, to be specific. Of course some "letters" are hardly recognizable as letters for many people (for example, Vai syllabary) but they are used as letters in at least one script.
There are a few in the higher planes that may give you problems (for example phoenician etc..). It is also important that the Browser does not do a replacement. You need a font like code2000 or latest version.