Sometimes the URL returned from computedStyle.getPropertyValue can be an empty string, rather than a URL. So we need to wait for the value to be a url string. Differential Revision: https://phabricator.services.mozilla.com/D220804
Sometimes the URL returned from computedStyle.getPropertyValue can be an empty string, rather than a URL. So we need to wait for the value to be a url string. Differential Revision: https://phabricator.services.mozilla.com/D220804