Does HTML rendering differ amongst different versions of windows ? For example does firefox 3.5 render HTML on xp the same as windows 7 ?
Not so sure about rendering, but fonts may be different. Font smoothing is better I think on > Windows XP, and the list of default fonts may be larger on newer versions (however keep in mind that by using these newer fonts, you should provide a fallback font)
It's up to the browser to parse the HTML. Browsers don't rely on the operating system to render HTML; they do it themselves using their own rendering engines.
However, there can be limitations in an OS that would prevent a browser from running on it. For example, an older version of Windows might not be able to run the latest version of Firefox. Also, now that some modern browsers can take advantage of graphics acceleration, the pages they render might look different in older versions of Windows where advanced graphics is not supported (i.e. the latest version of DirectX can't run on older versions of Windows).
Yes and no respectively. While the rendered pages may appear the same or with some noticeable changes, rendering varies, at least on a lower level perspective.
You might want to check,
I've never noticed a difference between XP Sp3 and Win 7 with the same browser. I imagine some of the fonts may be different but beyond that I doubt there would be any noticable difference.
Even between windows 7 and a Mac I have only ever noticed font differences between the two in Firefox or Safari.
The real differences you have to worry about is all the different browsers. The difference between operating systems seems to minimal except for fonts.