JavaScript to copy all CSS to a DOM-created element, except where defined in external stylesheet

Question

I need to be able to copy all of the CSS properties from an existing HTML element to DOM-defined element, except where CSS properties are already defined in an external stylesheet for the DOM element.

I have an external stylesheet:

#MyTargetElmnt { font-weight: bold }

This works perfectly for copying the all CSS properties:

objSource = document.getElementById('MySourceElmnt');
objCSS = window.getComputedStyle(objSource);
objTarget = document.createElement('div');

for (var i = 0; i < objCSS.length; i++) {
  objTarget.style.setProperty(objCSS[i], objCSS.getPropertyValue(objCSS[i]));
}

objTarget.setProperty('id', 'MyTargetElmnt');

...but, if the source element has a style of font-weight: normal, that's what will stick.

I think what I need is something like:

...
objTarget.setProperty('id', 'MyTargetElmnt');

for (var i = 0; i < objCSS.length; i++) {
  if (!objTarget.style.getPropertyValue(objCSS[i]) && !objTarget.style.hasOwnProperty(objCSS[i])) {
    objTarget.style.setProperty(objCSS[i], objCSS.getPropertyValue(objCSS[i]));
  }
}

I'm stuck on what to put inside if ()

I dumped the objTarget.style object into console.log and it does not contain any of the CSS properties defined the stylesheet. style.getPropertyValue is the same and I can't use getComputedStyle on the target element for comparison because it will contain all of the computed styles that I want to copy from the source element (which will cause it to copy no styles at all).

How do I obtain the CSS properties from the external stylesheet for comparison?