To play the devil's advocate: so that you notice that something is amiss and you fix it. The problem with margins on these elements (at least, this used to be t

To play the devil's advocate: so that you notice that something is amiss and you fix it. The problem with margins on these elements (at least, this used to be true - not sure if this is fixed nowadays) is that every browser implements its own defaults. Some of them might be suitable, but that will only mean that if the designer doesn't set them, these settings will be off in other browsers. So someone who cares might just as well set everything to 0 and then set as needed. Also, less likely that some margin will be set where 0 is appropriate. On the other hand not much is lost with this. If css is ignored, cool. If not, other settings will fix layout as needed.