Why is IIS failing to read the <system.web.extensions> section even though ASP.NET 4.5 is installed?
IIS fails to read the <system.web.extensions> section in your web.config—even after ASP.NET 4.5 is installed—because IIS doesn't automatically know how to parse custom or extended configuration sections unless they’re explicitly declared in the <configSections> block at the top of your web.config.