Not long ago, it was believed that Google would prefer m.sites because they were specially optimized for mobile users, but that's certainly not the case today. Big G states "Google does not favor any particular URL format as long as they are all accessible to both Googlebot and Googlebot-Mobile."
The above link mentions the 2 most common misconfigurations: faulty redirects and smartphone-only errors.
Google reports it's a common error for sites that employ user agent detection and redirection to pages on a mobile domain to send users to the wrong URL, such as the home page. Because the user experience is so poor when you don't deliver the right page, Google advises it's better to redirect to a desktop page than to an irrelevant page.
Peter McLachlan, chief architect at Mobify cautions that using proxy vendors that rely on device databases contribute to these errors, as these databases are often incomplete, and there is no authority that mandates device manufacturers register with them. With thousands of devices in users' hands, going responsive is a safer bet for new and fringe devices.
404 pages are painful, but shutting mobile users out of content that exists on desktop is downright cruel. Common snafus include incorrect handling of Googlebot-Mobile (sending it to the feature phone page instead of smartphone, redirecting to desktop, creating an infinite loop of redirects), and (headsmack) making the smartphone-friendly page itself a 404.
But wait, there's more...Google hints that you should care about all their recommendations for optimizing the mobile experience.
Some video can't be played on smartphones (cough, Flash on iPhone). Google suggests standard HTML5 tags for video, steering clear of formats that are not supported by all mobile devices.
App download interstitials
It's tempting to pimp your app to visitors that hit your site from a smartphone, and it can be smart. But when your request is a splash page, or even lightbox overlay, you risk turning visitors (and Google) off.
Some sites use the less disruptive method of showing a banner above or below content. So long as it's not disruptive to users, Google's cool with it (and recommends it).
Bad linking will get you in SEO trouble, and mobile offers a new way to mess up. Similar to the redirect issue, a common glitch is when a website allows users to switch from the mobile version to desktop, and vice versa, but points the link at an irrelevant page, like -- you got it -- the home page. Google crawls links. It'll catch you if you do this.
Slow loading pages
Just like on desktop, every second counts (though mobile users are reportedly willing to wait longer than the 2 second threshold for desktop, mobile users are often using flakier connections). While redirection can slow down page load, responsive sites can be slow if they're not optimized. (Heads up: next week we'll look at 5 ways to ensure your responsive pages are loading lightning quick).
Why Google recommends responsive design on smartphones and tablets
1. A single URL makes your pages easier to link to, share in social networks, email to other users and bookmark with tools like Google Bookmarks for users, and for Google's algorithms to assign indexing properties to.
2. Google's goal is to deliver the best quality results to users at all times. User-agent detection and redirection is not foolproof, and can degrade your site's user experience and increase page load time (though responsive sites, if not optimize for page load speed, can also frustrate users).
3. Googlebot has less work to do when your content lives on one URL. Rather than crawling your site multiple times with different user agents (and at different times), Google can efficiently keep up with your site, and keep indexed content consistent across devices.
Igor Faletski, CEO of Mobify suggests a fourth reason for a one-URL approach -- PPC. Google Adwords now treats tablets and desktop as one, leaving smartphones as the only device left in the mobile bucket. One URL is easier to maintain and scale in paid search campaigns.
To be clear, Google will not penalize you for not using responsive design. You may not even be penalized for doing one or more of the above (there's a difference between a penalty and just having less ranking power). It's also important to note Google doesn't play favorites with responsive sites, it recommends responsive because in some ways it's better for users and search bots. If you're not using responsive, maximize your mobile SEO by correcting any misconfigurations your mobile set up may have.
Join Linda Bustos at the Shop.org Merchandising Workshop July 16 for Mobile Commerce: Is Responsive Right for Retail?