Using the HttpHandlers and HttpModules request is first passed through the Share Point runtime and then passed to the ASP.NET runtime (aspnet_isapi.dll).
Common pages are stored on file directories while customized versions of pages are stored in content database.
Customizing SharePoint Web Parts with Custom Properties
So when a page is requested it first checks in the document table and then goes to the content table to load the page. If it does not find data of the page it goes to the file directory to load the page. This loading is done by ASP.NET runtime himself. But if there is data present in the content table then it’s loaded by the ‘safe mode’ parser.
How to create a Coded UI Test