When trying to browse web sites using Safari on a Mac, the pages from a number of different sites were poorly formatted with few if images shown. In some cases the site wouldn't load at all and Safari reported it was unavailable. Further investigation also showed the Mac reporting that it could not connect to the App Store.
The problem related to the cache of certificates that OS X keeps. Once the cache was cleared (deleted) and the Mac restarted the pages displayed correctly and the App Store was available again.
To clear the accepted certificates cache:
For further details see this Apple Support Article