In days past web designers created web sites that worked well on desktop computer monitors. Today with the proliferation of smart phones and tablets it is essential web sites be designed to allow viewing on those devices. There are four web design approaches to structure: Plain HTML, Responsive, Mobile and Apps.
Plain HTML structure web site design comes with every basic old school web site. Super easy and simple. $
Responsive structure web site design provides an optimal viewing experience — easy reading and navigation with a minimum of re-sizing, panning, and scrolling. $$
Mobile structure design creates web pages that fit smart phone screens. These pages are optimized for viewing the elements essential to the viewers experience with minimal fluff. Most web sites with mobile pages are designed to detect if a mobile device is accessing the web site and will redirect to pages designed for mobile devices. $$
Apps are programs designed to be downloaded and installed on mobile devices. Most of these programs will access data on the Internet and or other programs. This type of application requires more technical expertise and sometimes complex licensing requirement especially with Apple. $$$$
WordPress themes are now being designed with the ability to be responsive. Any user testing should include re-sizing the screen and or testing using smart phones and tablets.