.NET - How to fix error Message 'Sys.WebForms.PageRequestManager' is null or not an object

Error message: Message 'Sys.WebForms.PageRequestManager' is null
or
 not an object or 0x800a138f - JavaScript runtime error: Unable to get property 'PageRequestManager' of undefined or null reference


watch on YouTube:
while trying to execute the following code: Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(doSomething);

To fix it:

 In web.config replace  
   <system.web>
    <xhtmlConformance mode="Legacy" />
with 
  <system.web>
    <xhtmlConformance mode="Transitional" />