If you want a professional, open source CMS, I would strongly recommend TYPO3. See them at TYPO3.com, and the user association is TYPO3.org.
It takes the implementer time to learn, but is dead-easy for the users. Look at Dassault Systemes site at http://www.3ds.com. It's in 10 languages and has many thousands of "pages." Or look at http://www.bianchiusa.com/ for a flashy consumer products site.
I have used it for sites as small as 10 pages, so clients can grow the site easily as they grow their business, and for much larger sites.
It is open source, and is on version 4.1.2 with version 4.2 in testing. The lead developers are working on version 5. There are 2,600+ enhancements that can be installed from the repository.
It will output pure CSS code that meets W3C standards for HTML and CSS.
Regards -- Tony Lush
Advance Web Marketing
Industrial Strength TYPO3 Content Management