Partial update does not work or server requirements for AJAX

Actually we have already found the solution but it have confused us even more than original problem.

We have decided to checkSupportPartialRendering property of ScriptManager object. So I have added a label on the page and make it show the value of that property and the value ofEnablePartialRenderingon PageLoad event.

The result was:

EnablePartialRendering:True, SupportPartialRendering:True

on my local machine but

EnablePartialRendering:True, SupportPartialRendering:False

on remote server.

After that we read inAJAX documenation for that this property must be true when the following conditions are met:

TheW3CDomVersion property value must be greater than or equal to 1.0.

The problem was caused by <xhtmlConformance mode="Legacy"/> setting in machine.config (possibly) or somewhere else in server configuration.
Sincewe do not have this setting in our Web.config file - it was taken fromserver configuration. Once I had added the following tag:<xhtmlConformance mode="Transitional"/> into Web.config file ourweb application had started to work correctly.