JQM adds tabindex=0 - what is this for?

JQM adds tabindex=0 - what is this for?

In JQM v1.4.2, on line 4410, in $.widget._enhance() there's a line of code:

this.element
    .attr( "tabindex", "0" )
    .addClass( "ui-page ui-page-theme-" + this.options.theme );

Can someone explain the reasoning behind adding the tabindex?