A potentially dangerous Request.Form value was detected from the client (Login1$txtUserName="<html>").Description: Request Validation has detected a potentially dangerous client input value, and processing of the request has been aborted. This value may indicate an attempt to compromise the security of your application, such as a cross-site scripting attack. To allow pages to override application request validation settings, set the requestValidationMode attribute in the httpRuntime configuration section to requestValidationMode="2.0". Example: <httpRuntime requestValidationMode="2.0" />. After setting this value, you can then disable request validation by setting validateRequest="false" in the Page directive or in the <pages> configuration section. However, it is strongly recommended that your application explicitly check all inputs in this case. For more information, see http://go.microsoft.com/fwlink/?LinkId=153133.
Exception Details: System.Web.HttpRequestValidationException: A potentially dangerous Request.Form value was detected from the client (Login1$txtUserName="<html>").
Insert the following lines in your web.config
//If your using .NET 4.0 use the following attribute
<httpRuntime requestValidationMode="2.0" />