Mr. Harker said that even with responsive design ? which is the creation of websites that automatically optimize themselves on both desktop and mobile devices ? mobile Web pages still load too slow to deliver an acceptable experience for consumers. As an alternative to responsive design, marketers can choose to either design two different websites for desktop and mobile, or outsource the conversion of their desktop site into one that is optimized for mobile. 

?If responsive Web design is going to succeed, the top thing you need to consider is how do you make it acceptable to the consumer on smartphones,? Mr. Harker said. ?You can make it acceptable on desktop and tablet pretty easily.? 

Consumers will not tolerate page loads that take longer than about two or three seconds, he said. He cited a Keynote study of responsive design Web sites that found that even the best sites took several seconds longer than that to load. The slowest responsively designed Web site that Keynote studied took more than 46 seconds to load. 

?You need to think about milliseconds, not seconds, when it comes to page load times on mobile,? Mr. Harker said. 

Among the benefits of responsive Web design are that there is only one URL for each page on the Web site. In addition, social-media engagement is enhanced. 

?If someone links your page to Facebook or pins it on Pinterest from a mobile device, and the person who looks at it is on a different type of device, it doesn?t make a difference with responsive Web design,? Mr. Harker said. 

Another advantage is that responsive Web design allows for faster innovation, because development teams are not as specialized. 

?You only have one team focused on building your site, not two teams or three teams,? Mr. Harker said.

Responsive design can also improve search engine optimization because only one version of each page will exist, rather three or four. 

One of the major challenges of responsive Web design is that many ad brokers cannot deliver ad images that are fluid. Instead, they serve content at fixed pixel dimensions, which complicates delivery to smartphone browsers. 

In addition, the initial development of a responsively designed Web site can take longer than it would take to design either a mobile site or a desktop site. There are more requirements in the planning stages, and responsive design requires more testing time. 

Mr. Harker said that on most sites built with responsive Web design, marketers use a process called ?graceful degradation.? In that approach, the site is designed to deliver the best experience possible on the best device and under optimal conditions, and then accommodate any shortcomings for sub-optimal devices or situations. 

Instead, marketers would benefit from taking an approach called ?progressive enhancement,? through which the best possible experience is designed for the most basic devices first, and then enhance the experience for devices with greater capabilities and fewer constraints. 

Another consideration is page weight, Mr. Harker said. 

?You can?t expect a home page that has 150 requests on it to load in under two seconds,? he said. ?It will not happen on mobile. Even though it may be visually appealing, you can?t have a site that takes 45 seconds to load. You cannot have that in mobile.? 

Ken Harker is a Austin, TX-based senior consultant at Keynote Systems