Does anyone have good examples of browsers that have documented quirks on different operating systems? Some quirks are just because of text size but I always feel like there are more out there.
James: that appears to be the text size issue... the first one is OSX and you can see that the fonts are rendered slightly larger and antialiased... where as windows has rendered the text with no antialias... it looks like that is causing the tabs to flow wrong.
To answer the question, the engines for rendering out HTML (ie: gecko, webkit etc) should be consistent across operating systems, however the elements that are pulled from the OS (ie: fonts) will be arbitrarily different...
I haven't figured it out yet but I have had Firefox 3.5 give me different results. Both of these are the same site, One in OS X 10.4 and one in Windows XP.
I've found that Firefox 3 on OS X renders many things (floats not clearing specifically) the same as Internet Explorer 6. I had a page where multiple floated divs that were causing a layout problem in IE6 and Firefox on OS X, but in everything else it looked fine.
One commonly overlooked area is Forms. These elements will render very differently on OSX, Windows and in various browsers. Special attention should be given to buttons and select menus. Text inputs are a bit easier to control.
An excellent resource for this is Christopher Schmitts very extensive http://www.webformelements.com Flickr set which includes cross-platform screen captures of just about every imaginable variant.