On every browser I've tried (Firefox, Safari, Chrome, and Omniweb), when I go to a web page containing somewhat less-common characters, I can't see the glyphs. For example, on the Wikipedia page for the Bengali Language, the very first line contains a string of squares; on Windows, I can see the Bengali writing. Firefox does display code points on the Coptic Language article, but not Bengali. I'm not sure why.
On Windows, as long as I have the Arial Unicode MS font installed, these characters fall back to that font and display properly. Mac OS X doesn't seem to ship with a font containing these Unicode characters (it has Arial Unicode MS, but it must be a subset of the Windows version because Bengali doesn't display in that font).
I checked on my Snow Leopard DVD and I installed "Additional Fonts" from the Optional Installs package, but I'm still missing many languages.
Is there any good, preferably free font that contains a large collection of languages? I know creating fonts is difficult and time-consuming, but it seems like including at least one font like this with operating systems should be standard by now.