I thought this was already possible in earlier versions of OS X but Yosemite seems to be missing the 'Web.qlgenerator' quick look plugin (or at least I am)
How do you enable .url quick look to see rendered html in Yosemite?
I found this question of someone who wants to disable the rendered html and have it show to source code instead, so it should be possible
Quick Look doesn't make web requests, so it can't show the contents of the webpage without opening it. The question you linked was asking about .html files, with the actual source saved locally. .url files only include a URL, so without downloading the page it can't render anything. You can still use .html or .webarchive files and they will be rendered in Quick Look (with a .webarchive everything, including images and styles, will be included).
