One Web?

There remains avid debate about whether it is theoretically possible to write one web site that will run usably on both desktop and mobile devices.

The "there is only one web" argument: if we could just get mobile user agents good enough, and site designers would use appropriate CSS, mobile devices could effectively display the same web content as desktop (or other) devices.

I disagree. In essence, the technology problem can be solved, but the content problem can not.

• Non-visual presentations - Content written for visual (text + graphics) display can not be automatically rendered by voice, and vice versa. While a good text-to-speech solution theoretically could be developed, the content would still need to be altered. People process information differently verbally and visually. Worse, they make navigation decisions differently when they have to remember everything compared with when they can see their choices.
• Content style - Style needs depending on medium as well as the content itself. For example, many good verbal presentations follow the "tell them what you are going to tell them, tell them, then tell them what you told them" formula; good journalistic writing has the "inverted triangle" style with the big story first and revealing progressively less important details. Blog writing tends to have lots of links and be in either essay style (like verbal) or brief style. The needs of desktop and mobile device presentation are generally different.
• User context and needs - While your users may be interested in your marketing sheets, in spending half the day on your site, or in downloading large flash presentations while they are sitting at their desk, they likely won't be looking at this information while mobile. Further, the user is going to be in an environment where interruptions must be considered normal and frequent. However, the information needs to be presented in alternate forms. On the other hand, most of your desktop users will not be interested in downloading ringtones to their computer.
• Device features - You can ignore the fact that the phone has location, an excellent readily accessible voice connection, and quick messaging such as SMS or MMS, but why hamper your mobile web site like that?
• Response modality - If I knew that I could allow the user to respond using voice - something not always possible or desirable - I would design the site differently. For example, I would not have "Add to Cart" as the button labels, as that is what the user would have to say. The list would have to be designed differently. However, in a non-verbal browser this label is perfect.

Lots of people talk about the risk of fragmenting the web, or at least continuing the current fragmentation. I'm all for ending that. Let's get the fragmentation down to a minimum:

1. Large-screen displays
2. Small-screen displays
3. Large-screen displays with voice response
4. Small-screen displays with voice response
5. Voice display and response only

I don't think the list is going to get any shorter than that.