…but may lead to unexpected results, especially is switching between “strict” and “transitional” doctypes.
I discovered this by chance very recently. One page on a site I was building included a Google map, so I temporarily changed the doctype to transitional, just to stop the validator showing me errors I already knew about. It took me ages to equate that change with the reason the spacing between my header image and my navbar vanished on that page, but that’s what it was.