At least on Firefox it is both. By default it uses raw fontconfig for default, serif, sans-serif and monospace. But it uses an override for emoji fonts. You can

At least on Firefox it is both. By default it uses raw fontconfig for default, serif, sans-serif and monospace. But it uses an override for emoji fonts. You can also explicitly override these in the preferences and then it doesn't consult fontconfig (or probably consults it with a precise font name). But yes, I have seen decent browser fonts for most Linux distros.