selection.js

/*
index.html
<div id="div" hidden></div>

style.css
#div {
    border: 1px dotted #000;
    position: absolute;
}
*/

var div = document.getElementById('div'), x1 = 0, y1 = 0, x2 = 0, y2 = 0;
function reCalc() {
    var x3 = Math.min(x1,x2);
    var x4 = Math.max(x1,x2);
    var y3 = Math.min(y1,y2);
    var y4 = Math.max(y1,y2);
    div.style.left = x3 + 'px';
    div.style.top = y3 + 'px';
    div.style.width = x4 - x3 + 'px';
    div.style.height = y4 - y3 + 'px';
}
onmousedown = function(e) {
    div.hidden = 0;
    x1 = e.clientX;
    y1 = e.clientY;
    reCalc();
};
onmousemove = function(e) {
    x2 = e.clientX;
    y2 = e.clientY;
    reCalc();
};
onmouseup = function(e) {
    div.hidden = 1;
};