Orthogonality of Specifications
The general principle of platform design is that platforms consist of a set of standard interfaces. Standard interfaces allow substitution of components across the interface boundary, while independence of interfaces allow evolution of the interfaces themselves. In a PC, for example, the disk bus interface allows many different disk vendors to offer disk products independent of the model of display or keyboard, but the orthogonality of interfaces allow evolution of the interfaces themselves. If the display interface were linked to the disk interface too tightly, it wouldn't be possible to evolve ISA to SATA without updating VGA.
In the web platform, the three important interfaces are transport, format and reference, and the current definitions of those interfaces are HTTP, HTML and URI. The interfaces are standard, allowing many different implementations: HTTP standard lets you use HTTP servers from many vendors, the HTML standard lets you use many different HTML authoring tools or template systems, and the URI specification allows identification of many different components.
While HTTP is the current "common denominator" protocol that all web agents are expected to speak, the web should continue to work if web content is delivered by other protocols -- FTP, shared file systems, email, instant messaging, and so forth. HTTP as it has evolved has severe difficulties, and designing a Web that only works with HTTP as it is currently implemented and deployed would unfortunate. We should work harder to reduce the dependencies and isolate them.
HTML is the 'lingua franca', the common language that all agents are currently expected to be able to produce, process, read and interpret (or at least a well-defined subset of it). Having a common language is important for interoperability, but the web should also work for other formats -- extensions to HTML including scripting, DOM APIs, but also other formats and application environments such as XHTML, Java, PDF, Flash, Silverlight, XForms, 3D objects, SVG, other XML languages and so forth. Certainly HTML has it has evolved is overly complex for the purposes to which it is designed.
The URI is the fundamental element of reference, but the URI itself is evolving to deal with internationalization, reference to session state, IRIs, LEIRIs, HREFs and so forth. Many applications use URIs and IRIs, not just the formats described above but other protocols and locations, including databases, directories, messaging, archiving, peer-to-peer sharing and so forth.
The is just one of many communication applications on the global Internet; for web browsing to integrate will with the rest of the distributed networking, web components should be independent of the application, and work well with messaging, instant messaging, news feeds, etc etc.
A sign of a breakdown of this architectural principle would be for a specification of a format (say HTML) to attempt to redefine, for its purposes, the protocol (say HTTP) or the method of reference (URI). The specifications should be independent, or at least, dependencies isolated, minimized, reduced. If those other elements of the web architecture are incorrect, need to evolve to meet current practice or have flaws in their definitions, they need to evolve independently, so that orthogonality of the specifications and reusability of the components are the promoted.
There may well be reasons to link some features of HTML to the fact that it is delivered over an interactive protocol, but linking HTML directly to HTTP in a way that features would work only for HTTP and not for any other protocol with similar features – that would be unfortunate. It might not matter in the short-term (that’s all we have right now) but it is harmful to the long-term evolution of the web.
(Should go without saying, but just in case: this is a personal post, not reviewed by the TAG)