More than 6 in 10 U.K. retailers have an iPhone app, and more than 4 in 10 offer Android apps, according to the Internet Advertising Bureau UK. Meanwhile, about a third offer apps geared specifically to tablet devices. In addition, about 75% of retailers have mobile-optimized sites. "[S]martphone and tablet users do not necessarily want to download the app of every retailer they might one day visit, online or in-store. This is why mobile-optimized sites are also critical," eMarketer says.