CSS rule to apply only if element has BOTH classes

If you need a progmatic solution this should work in jQuery:

$(".abc.xyz").css("width", 200);

Below applies to all tags with the following two classes

.abc.xyz {  
  width: 200px !important;
}

applies to div tags with the following two classes

div.abc.xyz {  
  width: 200px !important;
}

If you wanted to modify this using jQuery

$(document).ready(function() {
  $("div.abc.xyz").width("200px");
});

div.abc.xyz {
    /* rules go here */
}

... or simply:

.abc.xyz {
    /* rules go here */
}