Issues on Sierra with Quicklook and website URLs. If you drag and drop a URL from Safari to the desktop (weblock icon) and use quicklook on the icon to view the website, Finder crashes immediately. Likewise, URLs imbedded in emails are not viewable with quicklook. There were similar Quicklook-related issues with Safari 11 where in the previous examples, using quicklook would only produce a Safari icon which when clicked would open the URL in Safari. In all other respects Quicklook works as expected - so long as the file is not dependent on Safari. Safari 10.12.2, which was installed with last update to Sierra (10.12.6) works fine with Quicklook and eliminates all of the issues above.