The only major drawback I see is that it pollutes the tab history so that if you view several images you have to click back several times to actually go back one page. This is no fault of the author as that is the expected behavior of the browser but it is something to keep in mind if one wishes to use this. Most people have been trained into thinking that a popup gallery of this sort doesn't affect the back button so it may come as a surprise to them to have to keep clicking back.
Otherwise, it's a clever use of :target and classes.