Horizontal web templates were a fad circa 2002ish...not so much with commercial websites though. Still see a few around in people's portfolios occasionally, but I personally don't think they are going to take off. They're a pain for mobile and tablet users, a pain to code, and a lot of people just don't 'get' them. Limited by resolution in terms of vertical length as well, so best suited for those who don't have much info and don't need to think about SEO.