Can you add Concrete5 as a CMS to a site already built with HTML5 and CSS?
I have a couple websites done already and I don't want to redo them - I simply want to add a CMS that will allow clients to update certain areas of a site already built with HTML5.
If you don't want to go through this trouble, you might want to try "Perch" (http://grabaperch.com/ ), which is a very simple CMS that *is* easy to drop into an existing HTML site -- but the downside is that its feature set is very limited compared to Concrete5 (but it might be enough features for you, so I'd check it out and see).
In particular, .htaccess tests to see if any http request maps to an existing file and simply serves the file, whether it is a picture, script, or html page. Only if the file does not exists does the C5 dispatcher get called.
With that in mind, I suspect you will need to move the home page into C5. The rest could coexist within the same web space, though you may have to install C5 into a subdirectory if any of the directory or filenames clash.
The rest is as previously noted, creating a C5 theme matches your co-existing html theme. For links between the two, in C5 you can code them as external links.