Websites are dynamic now, and have completely different experience on each device like iPhone, android phone, tablet, ipad etc. So the website should be compatible to each device. So we should know that how search engines like Google supporting the smartphone optimized websites.
Google supports smartphone-optimized sites in 3 configurations:
1. Websites that use responsive web design, i.e. websites that function all devices on the same set of URLs, with each URL serving the same HTML coding to all devices and using just CSS to change how the page is rendered on the device. Recommended by Google.
2.Websites that dynamically serve all devices on the same set of URLs, but each URL serves different HTML (and CSS) depending on whether the user agent is a laptop or a mobile device.
3. Sites that have different mobile and desktop URLs.
Google recommend using responsive web site design because it has many decent features:
1. Using same URL for a piece of content makes it easier for your visitors to interact with your content, and a single URL for the content helps Google’s algorithms allocate the indexing properties for the content.
2. There is no redirection is needed for users to get to the device-optimized view, which reduces loading time. Also, user agent-based redirection is error-prone and can degrade your site’s user experience (see “Pitfalls when detecting user agents” section for details).
It saves resources for both your site and Google’s crawlers. For responsive website design pages, any Googlebot user agents needs to crawl pages once, as opposed to crawling multiple times with different user agents, to retrieve your content. This improvement in crawling efficiency can indirectly help Google index more of the site’s contents and keep it appropriately fresh.