The HTML 4.01 specification has an IMG element, but there is no normative dependency on the PNG or GIF or JPEG specifications. “What good is an HTML user agent that doesn’t support GIFs?!?” you might ask. And you wouldn’t be alone. From the early days of W3C, there have been calls for a standard “web browser profile” component specification that listed which URI schemes (http, ftp, mailto, …) and which formats (HTML 3.2, GIF, …) and so on a standard web browser should support. It always seemed to me that the market would sort that out by itself and any standard, W3C could put in place, would be perennially out of date and irrelevant.
When two specifications are orthogonal, one may change one without requiring changes to the other, even if one has dependencies on the other. For example, although the HTTP specification depends on the URI specification, the two may evolve independently. This orthogonality increases the flexibility and robustness of the Web.
W3C inherited from the IETF a bias for specifying interfaces rather than components; i.e. data formats and protocols rather than software modules. I gather that in TV/consumer electronics, there are useful component standards for Web User Agents. But note that an in-car voice browser or a screen reader or good old lynx doesn’t support PNG nor GIF, and while their marketplaces are perhaps smaller than desktop or mobile screen-oriented browsers, they’re pretty much first-class citizens as far as W3C specifications, especially Web Architecture, are concerned.
APIs are more like interfaces than components, but they tend to be tied to platforms. The IETF has a cultural bias for on-the-wire formats over APIs, and for good reasons, I think. With OMG specs, at least the early CORBA specs, lots of products conformed to the spec (or at least claimed to) without actually interoperating with each other. It wasn’t until the arrival of IIOP, an on-the-wire CORBA format, that the rubber hit the road and the interoperability issues got addressed.
There are precious few “W3C should never do XYZ” rules that I think are worth setting in stone. While we will naturally attract work that is like what we have done before, any place we can get a critical mass of the marketplace to get together and do the hard work of testing, internationalization, accessibility in a reasonably timely, fair and accountable way is a place where W3C should be able to do more good than harm.