However, those browsers, even at the same versions, may not always behave identically on different platforms.
Netscape is available on more platforms than MSIE. Opera and iCab are likely to be available as widely as Netscape, but both are stressing their adherence to the standards and both make bold claims to be compliant. That suggests that their behavior across computing platforms will be identical.
In actual fact, there are minor differences even on the best crafted browser projects. MSIE and Netscape fall some way behind the lesser known browsers in this respect.
It is possible to install a variety of browsers onto a single Macintosh system. It is a little more difficult with Windows installations, which generally do not cope well with more than one version of any particular product being installed. You experience difficulties installing older versions of MSIE onto recent versions of Windows that already have an embedded web browser.
You could, if you are serious about cross platform testing, easily end up with a test suite of 10 or more machines. That way you would likely be able to cover Linux as well.
Although you may install many browser applications on a single Macintosh, you may want to add more hard disks and install multiple operating system versions so you can multiple-boot the machine.
This is because some cross-platform differences are due to underlying operating system level code, which may be revised with a new release of the OS and which may manifest itself as an artifact on the web page when it is displayed.
The navigator property tells you a lot about the browser and platform. However, MSIE supports other platform and browser detection methods that are not available on other browsers.