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Google, Bing and Yahoo all have different search bots for mobile. Google’s recommendations on how to make sites mobile friendly are all based around building separate sites and then ensuring that the Google mobile bot is redirected to the mobile version. Matt Cutts has talked about this in a video answer and Google has gone so far as to describe how you should redirect the Google mobile bot in some detail.

What does this mean for sites using responsive web design? Honestly, I don’t know. Nor does anyone else it appears. There is some question about whether or not mobile SEO is even worth pursuing.

But it is worrisome. Google recently announced that mobile website optimization now factors into mobile search ads quality. I’ve seen no indication that Google is considering responsive web design in its definition of mobile optimization. The announcement linked to several case studies and articles illustrating separate sites as the approach.

This is probably a problem with the search engines and not with responsive web design as a technique, but if a company relies on search engine traffic for revenue, it likely won’t matter who is to blame.

Advertising

But there is another more structural issue: sometimes advertisers just want to advertise on one form factor and not another. App developers who want to drive app store purchases may not be interested in advertising on desktop. An advertiser interested in location-based advertising is also unlikely to consider responsive advertising desirable.

Will responsive web designs be able to participate in the growing mobile advertising opportunity?

Analytics

Most web analytics tools that support mobile provide an option to use a server side way of tracking instead of tracking via JavaScript. This is offered because many older devices either don’t support JavaScript or support it so poorly that using JavaScript-based tracking code is problematic.

Bryan and Stephanie Rieger have talked about how in their experience switching to the server-side code will show a lot more mobile traffic from a wider variety of devices than if you stick to the JavaScript version.

The problem is that you can’t run both the JavaScript and the server-side (mobile) variant on the same page. The analytics vendors recommend using the server-side code on your mobile site and the JavaScript one on your desktop site because the JavaScript version has a lot more data and features than the server-side one.

Sites using responsive web design will need to choose between more accurate data about the total mobile traffic (server-side tracking code) or deeper information about a small set of visitors (JavaScript tracking code).

Content Delivery Networks (CDNs)

On a responsive web design that takes care to provide only the assets that are appropriately sized for the device requesting the page, the images will vary based on the device. This causes problems with content delivery networks that have become accustomed to being able to cache a single asset for all devices or at worst caching a desktop and a mobile version of the asset.

As Ronan Cremin put it, CDNs may now need to cache different images based on the entire spectrum of devices accessing a site. FWIW, depending on how they are implemented, this can be a problem for separate sites as well.

One way in which RWD does not resemble the transition to standards

The move towards responsive web design has been compared by many to the changes that happened when web design went from being table-based to using web standards and CSS layouts. The Boston Globe’s new site has been compared to the impact that the ESPN and Wired redesigns in proving the value of web standards.

It may be that my faulty memory, but I don’t remember a similar list of potential drawbacks from a business perspective to making the change to CSS-based layouts. If anything, things standards-based layouts had great business benefits because of the increased semantic markup leading to better search rankings and the leaner code helping with page performance and bandwidth costs. Yes, like responsive web design, retraining staff and changing infrastructure could be a major undertaking, but there wasn’t concern that making a change would negatively impact search rankings or make analytics more difficult.

And I honestly don’t know how big of a problem any of these things are with the exception of the analytics problem which seems clearly to be a pickle. It may be that search engine rankings aren’t impacted at all. But the fact that we don’t know seems like something that should be sorted out and considered if those things are critical to the success of a given project or client.

And to be 100% clear, I’m not saying people shouldn’t do responsive web design. Even if you were to build separate sites, you should still do responsive web design for the separate sites.

I’m simply saying these are challenges and concerns that I don’t think we’ve currently got good answers for.”