Applied to: Windows Hosting Plan developer should be aware about the aspnet_client folder in their web space. This folder related to runtime validation and has its own importance. This folder automatically created when you enable the extension on your site. The aspnet_client folder contains the client-side code used by some of the ASP.NET Web Form controls (namely validation, SmartNavigation). The scripts contained in this folder get called by web pages that utilize controls that need these scripts. If you use a control that needs one of the scripts in this folder and the script can't be found, then that control will not be able to do its client-side job.  

 The aspnet_client folder should be considered as the ASP.NET runtime component not your ASP.NET application. The scripts in the folder might change in the new version. This is the reason why Publish Site tool will delete it first and re-create it. Consider if you have a web application develop in an older version then publish it to a web site with a new version of ASP.NET, the aspnet_client should have the scripts from the same version of ASP.NET. So it is good practice to left ASP.NET to maitain this folder.  

 If your aspnet_client directory is missing, you may encounter errors with your application such as "Unable to find script library '/aspnet_client/system.web/1_0_3705_272/webUIValidation.js'. To resolve this you can manually copy the folder to your root directory of your domain name.