What happens when your master page doesn't meet the site definitions?
Will it still render correctly?
Currently my master has the blue 'i' icon and doesn't meet site definitions. But still looks okay.
SharePoint pages are, by default, associated with site definition files. When a master page is customized in SharePoint Designer 2010, it is no longer associated with the site definition. This can result in additional administrative work following future updates or upgrades to SharePoint, which is why you see this message. You can always reset the master page to its site definition, if necessary.