I know this project has a disclaimer about not supporting all of the CSSOM, but I encountered some errors while trying to get HTML Inspector to work with jsdom instead of PhatomJS, and I tracked it down to the lack of these two properties on the CSSStyleSheet object.

From what I can tell, the following function in level2/style.js is where this could/should be added, but I'm not familiar enough with the codebase to know for sure.

function evaluateStylesheet(data, sheet, baseUrl) {
  // this is the element
  var newStyleSheet = cssom.parse(data);
  var spliceArgs = newStyleSheet.cssRules;
  spliceArgs.unshift(0, sheet.cssRules.length);
  Array.prototype.splice.apply(sheet.cssRules, spliceArgs);
  scanForImportRules.call(this, sheet.cssRules, baseUrl);

  // Add references to `this` and `baseUrl`.
  sheet.href = baseUrl;
  sheet.ownerNode = this;

  this.ownerDocument.styleSheets.push(sheet);
}

Anyway, this changed work for me locally, so hopefully it's as simple as that. Thanks for the consideration.