.style.width
will give you the width of the element, there is no .value
property for it.
it should be
var framewidth = document.getElementById('frame').style.width;
Demo: Fiddle
You might also want to have a look at window.getComputedStyle()
function frame700() {
var el = document.getElementById('frame'),
style = window.getComputedStyle ? window.getComputedStyle(el) : el.currentStyle || {};
var framewidth = style.width;
document.getElementById('frame').style.width = '700px';
document.getElementById("info").innerHTML = framewidth;
}
Demo: Fiddle