Preferences: Arial 16.
CSS: *, body, td {font-family:Arial,sans-serif;}.
Arial 16px as CSS - xx-smallll - x-smallll - smallll - mediummm - undefined.
Browsers:
NS = Netscape
IE = Microsoft Internet Explorer
OP = Opera

Below is the result in Windows browsers with preferences above (size values are in pixels; The yellow background color express the undefined font size, if the browser has at least at some level consistent implementation to it).

size NS 4.04 NS 6.0 OP 3.62 IE 5.5 OP 5.0 OP 5.10 OP 5.0 default
xx-small 8 9 9 10 15 13 10
x-small 11 10 11 13 18 18 13
small 13 13 13 16 22 22 16
medium 16 16 16 19 25 24 19
large 25 19 19 23 32 32 23
x-large 36 23 23 32 45 43 32
xx-large 54 32 28 48 64 64 48

In the following list, I rank the consistency and following of the CSS2 recommendation concerning relative font sizes:

  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 preview - quite consistent behavior (one test version worked as well as Opera 3.62). Medium is the same as undefined size. It also matches to the user setting of the default font size.
  3. MS IE 5.x and Opera 4.02 - 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. Different versions rounds font-sizes a little bit differently.
  4. Opera 5.0 ja Netscape 4.04 - very inconsistent behavior.

The 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. With default preferences it renders relative font size approximately at the same way as MS IE 5.5, but overall it works a little bit more inconsistent way than in MS IE 5.x.

Note. Microsoft will change in IE 6.0 the handling of relative font sizes so, that medium is the same as undefined font size (CSS Enhancements in Internet Explorer 6 Public Preview).

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

size NS 4.7 Mac MS IE 4.5 Mac
xx-small 6 9
x-small 8 10
small 10 12
medium 12 14
large 18 18
x-large 28 24
xx-large 40 36

Neither Mac browsers work consistent. MS IE follow approximately the same calculation principles between font sizes as Windows version, but a little bit an-exactly. If font small is 12px, the series should be in my mind: 8, 10, 12, 14, 17, 26, 38/39.