I would justify that by pointing out that web pages tend to scroll vertically; even (especially?) in landscape aspect, the rendered page is usually taller than

I would justify that by pointing out that web pages tend to scroll vertically; even (especially?) in landscape aspect, the rendered page is usually taller than it is wide. If you really need to just design the first screenful of a page in landscape orientation, then go ahead and draw a big horizontal line through the centre of the page -- label it "FOLD" if it helps, and just work on that screen.