We can get style property of an element via getComputedStyle() but is there a way detect/track where this value was set from: user's CSS or system default or neither?

In this example only bakckground-color is set by user's stylesheet, display is set by "user agent stylesheet" (aka browser default) and width is not set by anything, it's automatically generated:

console.log("backgroundColor", getComputedStyle(test).backgroundColor);
console.log("display", getComputedStyle(test).display);
console.log("width", getComputedStyle(test).width);

#test
{
  background-color: lightblue;
}

<div id="test">test</div>

p.s. in DevTools there is a link to the stylesheet where the property is set:

and it shows which stylesheet sets the value: