I have a webfont that doesn't support Cyrillic. The font only has latin chars. If I try and display some Cyrillic chars, instead of displaying blank or corrupt chars (as I would expect it) it bypasses the webfont and displays them in a system font instead. Whilst this is a good thing I don't know why or how. As I understand it, if a font specified in the CSS isn't supported by a browser or the browser doesn't support webfonts it is then that a substitute font is put in place. However this is happening if the characters used aren't supported. Is my understanding of this correct?
Most browsers do missing character fallback by default, so you never see blank characters when something's missing. If the developer uses a font family stack like font-family: MyCyrillicFont, Arial, sans-serif your browser will use the next available font. Otherwise, the browser itself has a default font setting (that the user can typically choose).
In short, browsers have fallbacks on the characters level.