Basically, it needs a font rendering engine that is both poweful enough to display lots of combined diacritics from various scripts in parallel and lenient enough to actually try and do it by stacking them way outside the line.

It might be that only Windows has such an engine; font rendering is certainly one of Microsoft's strength.

I know that it works in LibreOffice (does that leave font rendering to the OS?) and in Omegle on Firefox (what renders the font here? Flash? Firefox? Windows?).