Preferences: Arial 16.
CSS: *, body, td {font-family:Arial,sans-serif;}.
Arial 16px - xx-small - small - medium - undefined.
Browsers:
NN = Netscape Navigator
IE = Microsoft Internet Explorer
Op = Opera

Below is the result in Windows browsers with preferences above (values are in pixels and the aqua background color shows the calculated value according to CSS2).

Browser: Font size: NN 4.04 NN 6.2.1 IE 5.5 IE 6.0 (*) Op 3.62 Op 5.0 Op 6.0 Op 7.0 Beta1 (*)
xx-small 8 9 10 9 9 15 13 13
x-small 11 10 13 10 11 18 18 13
small 13 13 16 13 13 22 22 18
medium 16 16 19 16 16 25 24 22
large 25 18 23 18 19 32 32 24
x-large 36 23 32 23 23 45 44 32
xx-large 54 32 48 32 28 64 64 43
  1. Opera 3.62 - works exactly according to CSS2 (indeed the relative scaling factor can be changed). Medium is the same as the undefined font size, and in the German version it matches to the default font size (I some English Opera 3.6x the default font size matched to the font-size small).
  2. Netscape 6.x and MS IE 6.0 (in the standard-compliant mode, why I have used in the table above the '*' mark) - quite consistent behavior except the largest value (one evaluation version of Netscape worked as well as Opera 3.62). Medium is the same as the undefined font size. It also matches to the default font size of browsers.
  3. MS IE 5.x and MS IE 6.0 (not in the standard-complian mode) - partially inconsistent behavior. From xx-small to large goes according to CSS2 recommendation, but x-large and xx-large has 1.5 as the scaling factor. Medium is one step bigger font size than the default font size, which is the same as small. Also Opera 4.x works approximately at the same way.
  4. Opera 5.x+ and Netscape 4.x - very inconsistent behavior.

NoteThe most odd matter is between Opera 3.62 and Opera 5.0. Opera Software knows, how relative font sizes should be handled. But because MS IE works inconsistent, it doesn't know, what to follow - MS IE or the specification. Because the behaviour of MS IE 6.0 for Windows is DTD-dependent (CSS Enhancements in Internet Explorer 6 Public Preview), also Opera would need a DTD-switch.

I got an e-mail from MAC-user. Macintosh G3 PowerPC / MacOS 8.0 use smaller font sizes.

Browser:
Font size:
NN 4.7 MS IE 4.5 MS IE 5.0
xx-small 6 9 10
x-small 8 10 13
small 10 12 16
medium 12 14 16
large 18 18 24
x-large 28 24 32
xx-large 40 36 48

Neither Mac browsers work consistent. MS IE 4.5 for Mac follows approximately the same calculation principles between font sizes as MS IE 5.x for Windows, but a little bit an-exactly. If font small is 12px, the series of MS IE 4.5 for Mac should be in my mind: 8, 10, 12, 14, 17, 26, 38/39. DTD has no effect in this matter for MS IE 5.0 for Mac. Netscape 6.x for Mac behaves presumably at the same way as corresponding Windows versions. Then Netscape 6.x is the only browser in Mac, which behaves quite consistent.