The optional load-on-startup servlet element in a web.xml indicates that the associated servlet is to be loaded and initialized as part of the startup of the web application that declares it.
The optional content of this element is an integer indicating the order in which the servlet is to be loaded and initialized with respect to the web application's other servlets. An empty <load-on-startup> indicates that the order is irrelevant, as long as the servlet is loaded and initialized during the startup of its containing web application.
The Servlet 2.4 schema for web.xml no longer supports an empty <load-on-startup>, meaning that an integer must be specified when using a Servlet 2.4 based web.xml. If specifying an empty <load-on-startup>, as in <load-on-startup/>, the web.xml will fail validation against the Servlet 2.4 schema for web.xml, causing deployment of the web application to fail.
Backwards compatibility issue. Specifying an empty <load-on-startup> still works with Servlet 2.3 based web.xml.