Browsers will generally do "fallback" to some other font, if the font(s) named in the CSS don't support the characters present in the text. But for some of the rarer characters, you may not have any available font that supports them.
Since 1998 MacOS has a “last resort” font that has glyphs (not necessarily unique) for every Unicode code point. They donated it to Unicode (https://en.wikipedia.org/wiki/Fallback_font#Unicode_Last_Res...), so I expect most OSes running full-blown modern browsers to have it or something similar (those running smaller browser engines may be too space constrained to have room for it)