Default font set on Android
Expanding upon Grstmo's answer:
system_fonts.xml defines mapping from font-family to actual font files. So let's try these:
<div style="font-family: sans-serif; font-style: normal">Regular</div><div style="font-family: sans-serif; font-style: italic">Italic</div><div style="font-family: sans-serif; font-weight: bold">Bold</div><div style="font-family: sans-serif; font-weight: bold; font-style: italic">Bold-italic</div><div style="font-family: sans-serif-light; font-style: normal">Light</div><div style="font-family: sans-serif-light; font-style: italic">Light-italic</div><div style="font-family: sans-serif-thin; font-style: normal">Thin</div><div style="font-family: sans-serif-thin; font-style: italic">Thin-italic</div><div style="font-family: sans-serif-condensed; font-style: normal">Condensed regular</div><div style="font-family: sans-serif-condensed; font-style: italic">Condensed italic</div><div style="font-family: sans-serif-condensed; font-weight: bold">Condensed bold</div><div style="font-family: sans-serif-condensed; font-weight: bold; font-style: italic">Condensed bold-italic</div><div style="font-family: serif; font-style: normal">Serif Regular</div><div style="font-family: serif; font-style: italic">Serif Italic</div><div style="font-family: serif; font-weight: bold">Serif Bold</div><div style="font-family: serif; font-weight: bold; font-style: italic">Serif Bold-italic</div><!-- "Droid Sans" actually gives you Roboto. --><div style="font-family: "Droid Sans"; font-style: normal">Droid Sans Regular</div><div style="font-family: "Droid Sans"; font-weight: bold">Droid Sans Bold</div><div style="font-family: monospace; font-style: normal">Monospace Regular</div>http://jsfiddle.net/9z3tD/4/ => 
(tested on android 4.2)
EDIT: "Droid Sans" in the screenshot is clearly Roboto.The config supports it but turns out the underlying file is a simlink: DroidSans.ttf -> Roboto-Regular.ttf, DroidSans-Bold.ttf -> Roboto-Bold.ttf
EDIT: It doesn't seem to matter whether I quote the family names (http://jsfiddle.net/9z3tD/9/ renders the same). CSS distinguishes between generic keywords like serif and quoted "serif" which refers to a font whose name happens to be "serif"; but the xml config makes no distinction so I guess in android either syntax refers to a font whose name is "serif". But there may be a difference if you create a custom WebView and configure it to map generic keywords to other fonts (setSerifFontFamily() and friends).
Then there are fallback_fonts.xml. These have no names; e.g. you CAN'T say font-family: "Droid Sans Fallback".
But all of them are suppossed to be tried for every missing glyph in other fonts. Presumably one can even choose a subset using font-variant of "elegant" or "compact" and by setting text lang="ja"?
However I don't see Droid Sans Fallback being used at all so perhaps fallbacks don't work in WebKit :-(
Disclaimer: The above applies to stock Android, using stock browser and/or Chrome. I believe it also applies to WebView (with default config) but haven't tested.
In theory manufacturers may use other fonts, e.g. I heard rumors Samsung does it. In practice screenshots on all android devices BrowserStack had all — including Samsungs — look to my untrained eyes to be Roboto or Droid, the differences being just android releases...However here is Galaxy S3 caught not aliasing Georgia to serif so something differs.
Firefox is a good example of a browser doing it differently — they switched to Open Sans and Charis SIL Compact, then replaced Open Sans with Clear Sans.
The full config is in libpref/init/all.js and is language-dependent.
As far as I can tell, Firefox disregards the system_fonts.xml's aliases, e.g. Georgia results in a sans font.
OTOH, older versions (tested 30, 34) allowed you to refer to system fonts by name, even to Droid Sans Fallback but this doesn't work on FF 36, 37 (these 2 results are for http://codepen.io/cben/pen/VYgPEE, use of AdobeBlank means that when you see chars, that font was used).
See also relevant answer about fonts available to apps.