Consistency in a world gone Mobile
Thematic Consistency was introduced in the Mobile Web Best Practices 1.0, where it suggests you should: “Ensure that content provided by accessing a URI yields a thematically coherent experience when accessed from different devices.” The essence of this principle is that regardless of how you access something on the Web, you should get more-or-less the same result.
Obviously, the experience you get on a tiny mobile screen with no keyboard or mouse is going to be quite different from the experience you get on your big desktop computer. So what exactly is a “coherent experience”? To my mind, it is not the kind of One Web story that gets argued about elsewhere. Unfortunately the phrase “One Web” has been so badly abused that it is now meaningless.
A coherent experience is one that brings no great surprises. If you have an expectation based on a URI, that expectation should be honoured in a contextually appropriate manner.
Consider this simple example. If I have a site called example.com that offers train timetables, I expect to get train timetables when I make a request to that site. On my big desktop computer it is entirely appropriate for me to receive comprehensive timetables, colour coded, with all the fancy interaction we expect of Web 2.0. But on my small mobile Web-enabled device I would be happy enough to get even a piece of a timetable (with some navigation assistance to find other pieces). It might even be good to get by default timetables for trains at the nearest station within the next hour, which would be an entirely appropriate experience for a mobile user.
I would not expect to get a purchase interface for a flower shop on my mobile device, where on my desktop I get the train timetables. Such a situation would strike me as incoherent. This is an example of Thematic Inconsistency. It is this situation that the MWI Best Practices warn against.
It is not always so clear, however. If I have a site called example.com that redirects to mobile.example.com or desktop.example.com based on browser type detection, then I have an adapting site. Albeit a rather simple one. Suppose that mobile.example.com offers to sell you ringtones, because I know that the requesting device is capable of supporting ringtones and the connection is going via an operator who can help me bill for the service. Now also suppose that desktop.example.com offers to sell you hi-res animated desktop screen-savers. Is example.com following Thematic Consistency?
At first, it seems that in this example, Thematic Consistency is not being followed because if you access it with a mobile device you will reach mobile.example.com to purchase ringtones, and if you access example.com via a desktop browser you will get to desktop.example.com to purchase screen savers. The experience is different in each case, and arguably inconsistent.
Yet, I could present my business model, which is to sell novelty technical distractions for Web-enabled devices. Now it seems that mobile.example.com and desktop.example.com are consistent with the theme of my business, not necessarily with each other.
One can easily contrive many situations where Thematic Consistency is difficult to measure or verify. This is why I don’t believe it will be a testable part of any mobile site. It is certainly not part of the BPWG’s mobileOK. I have heard the argument that some trusted authorities could assess the content received by various devices in various contexts and assess if the results are consistent, and thereby validate any claims of consistency. (For a fee, no doubt.) Even then, different authorities may choose different criteria for such assessments.
Despite all this, Thematic Consistency (or the avoidance of Thematic Inconsistency) is still a worthwhile goal. Whether it can be achieved in practice will depend on many factors, not least the cost. And this is where I see the most difficulty, which is why I advocate that Thematic Consistency should be interpreted as Avoid Thematic Inconsistency. The latter is easier than the former, and it avoids a common misunderstanding: that every possible way of accessing a URI must present a Thematically Consistent response. I don’t think that was ever the intention of the practice, especially as such an interpretation would be an expensive (or impossible) objective.
On the other hand, Avoid Thematic Inconsistency does not require you to support every access mechanism, every network, every device… Instead, you are asked to ensure that for every way of accessing your URI (that you support) the response will be consistent with some theme.
This still leaves open the issue of discovering the theme, but perhaps this is where the Semantic Web could help. We could be waiting a long time.
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