look at this page : http://www.immobilzero.com/Scheda.aspx?CodImmobile=1 

it seems strange, but the headers are too short
i have set nothing to do this! (or i do not remember i done somethings)

how can i do for fix it?

What I have noticed is that you are applying a default size to the spans like this.

span {
	font-family : Verdana, Arial, Helvetica, sans-serif;
	color : Navy;
	text-decoration : none;
	font-style: normal; 
	line-height: normal;
	font-size: 11px;
}

This might be causing the behavior your mentioned with the tabs, since tabs are rendered as spans themselves. If you want to control a specific behavior of tab you can overwrite the following classes 

	ajax__tab_outer
	ajax__tab_inner
	ajax__tab_tab

Please check this link to define the custom css class for container.

http://blogs.visoftinc.com/archive/2007/09/26/ajax-control-toolkit-tab-control-themes.aspx