BUG: "Expected Identifier" Error Message on Default.aspx ASP.NET Page

The C# WebForm outputs illegal JavaScript to the client. The C# WebForm outputs an HTML form named "default" and then references that form with "document.default".

For example:

In JavaScript, "document.default" is an illegal usage of the keyword "default". Therefore, the browser displays a run-time error.

This illegal code is generated by any control that performs a PostBack.

Resolution

To work around this problem, rename the form in the Default.aspx WebForm by following these steps:

1. Open Default.aspx in Visual Studio .NET designer.
2. Click the HTML tab to view the HTML code.
3. Change the following HTML code
   to the following code:

Status

Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.

More Information

Steps to Reproduce the Problem

1. Open Microsoft Visual Studio .NET.
2. On the File menu, point to New, and then click Project.
3. In the New Project dialog box, click Visual C# Projects under Project Type, and then click ASP.NET Web Application under Templates.
4. In the Location text box, type the project name as TestApplication by replacing the default 'WebApplication#' name in the URL path.
5. Add a LinkButton server control to WebForm1.aspx.
6. On the File menu, click Save.
7. On the Build menu, click Build Solution.
8. In the Solution Explorer window, right-click WebForm1.aspx, and then click View in Browser.

https://support.microsoft.com/en-us/help/315990/bug-expected-identifier-error-message-on-default.aspx-asp.net-page