How to set up out-of-date HTML standard doctype in ASP.NET 4.0
The doctype directive occupies the second section in .aspx page or ASP.NET web forms and appears just after the page directive, described in the article How to use Page directive in ASP.NET 4.0
The directive indicates the type of markup, HTML, XHTML, etc., used in the web page. This directive is optional, but Visual Studio 2010 adds it automatically. …