A common question people often ask when hearing about the cleaner markup coming with ASP.NET 4 is “Great - but what about my existing applications?  Will these changes/improvements break things when I upgrade?”

To help ensure that we don’t break assumptions around markup and styling with existing ASP.NET Web Forms applications, we’ve enabled a configuration flag – controlRenderingCompatbilityVersion – within web.config that let’s you decide if you want to use the new cleaner markup approach that is the default with new ASP.NET 4 applications, or for compatibility reasons render the same markup that previous versions of ASP.NET used:

When the controlRenderingCompatbilityVersion flag is set to “3.5” your application and server controls will by default render output using the same markup generation used with VS 2008 and .NET 3.5.  When the controlRenderingCompatbilityVersion flag is set to “4.0” your application and server controls will strictly adhere to the XHTML 1.1 specification, have cleaner client IDs, render with semantic correctness in mind, and have extraneous inline styles removed.

This flag defaults to 4.0 for all new ASP.NET Web Forms applications built using ASP.NET 4. Any previous application that is upgraded using VS 2010 will have the controlRenderingCompatbilityVersion flag automatically set to 3.5 by the upgrade wizard to ensure backwards compatibility.  You can then optionally change it (either at the application level, or scope it within the web.config file to be on a per page or directory level) if you move your pages to use CSS and take advantage of the new markup rendering.