I've finally completed a working version of my first ever CSS-supported site (thanks to some very helpful suggestions on this forum) and have validated the CSS. Before I go out and find (buy?) another machine (I'm a Mac) to check IE on, is it remotely possible that the valid CSS is really truly valid on IE as well as Firefox, Safari that I have used to check my code so far? Thanks, Patrick.
Nope, W3C validates against the CSS standard. Unfortunately popular browsers (read: MSIE 6.0) are not very standard compliant ;)
I think it entirely depends on how complex your CSS and layout is - if its at all complex, then I wouldn't hesitate to say 'no'. Your best bet is to check it on other platforms.
You might want to look into BrowserCam - they do screen shots of websites in all major (and some minor) browsers, and the more expensive accounts allow you to use VNC to actually go interactive on those platforms.
"is it remotely possible that the valid CSS is really truly valid on IE as well as Firefox, Safari that I have used to check my code so far?"
Valid? As in accepted by a browser? Yes.
Look the same? Not necessarily.
If your page is simple enough and avoids all the known IE incompatibilities, you're good to go. [We try to do this: just avoid the weird stuff, use tables, accept the limitations of one-size-fits-all-browsers.]
My suggestion is that you put a "Download Firefox" badge on your page and not worry about IE.
Not all browsers fully, correctly, and consistently implement all CSS features, especially when it comes to CSS2 or the upcoming CSS3. W3Schools.com can help you, though, as it lists which features are compatible with which browsers—although I'm not sure how up-to-date this information is.
In the end, there's little substitute for testing on all popular browsers. I would recommend installing Wine, Parallels, or VMWare and testing IE and Chrome. It might be good to test Konqueror, as well, if that will run on OSX.
If installing one of these is too much a PITA (or if IE and Chrome won't run on Wine or Parallels—I'm not really sure—and you don't have a copy of XP to install on VMWare), you might have a friend running Windows that you could utilize. Worst case scenario, most libraries have Windows machines you could test with.
Remember that the validator can only see if your syntax is correct. It can not understand what layout you are trying to achieve, so that your page validates will never be a sign of it working as you wish in all browsers. It might do, but the validation itself can't tell you.
However, validating your pages will be a good idea nonetheless. If there are no syntax errors, the chances of everything working correctly grow.
To rescue the web designers from this aching job of testing browser compatibility in different browsers there are few websites which offer this service. On these websites you can check the compatibility of your website in all desired browsers. You can find these websites at http://www.bestpsdtohtml.com/7-awesome-resources-to-test-cross-browser-compatibility-of-your-website/