Nearly all specifications are created by a committee rather than by a single individual. The working groups of W3C typically consist of some 10 to 20 people, who work together on a new technology for a year or longer.
"Design by committee" has a bad name (specs that are a patchwork of inconsistent solutions, often redundant, and thus too big and too hard to learn), but in reality it doesn't automatically produce bad results. "Two know more than one" is another proverb, and that is exactly why working groups exist: more pairs of eyes mean more checking for errors, more creativity in finding solutions to problems, and more experience in knowing what worked or didn't work in the past.
But the problems of "design by committee" still have to be avoided. Around 15 people seems to be the limit, larger groups tend to form (informal) sub-groups and lose too much time in communicating rather than developing.
Smaller groups produce more consistent and easier to use specifications, but they may omit some things that they didn't know anybody needed. The solution seems to be to create a wider circle of interested people around them, in the form of a public mailing list.
That is how W3C develops its technologies: a working group recruited among experts, and a public mailing list for other interested people. There may be some there that have only interest in one detail or that only occasionally have time to discuss the developments. In the working group they would just have hindered the process, but on the mailing list they can give valuable contributions.
The IETF shows that it is possible to develop technologies with a single, open group of people, without distinguishing between people that have committed to a minimum amount of effort and those that haven't. But IETF groups tend to be about low-level, very technical specifications, whereas W3C specifications are (at least perceived to be) much closer to the average user and thus attract more interested people. It may also be that the environment has changed: the public on the Internet now is different from that before the Web.
Of course, the two-level system assumes a willingness of the committee to listen to the outside. It takes some time to scan the mailing list for important messages and, when needed, to answer them. But most of all it requires an openness on the part of the committee to discuss their reasons, even if they sometimes have to do with short-term company policies. And it requires a matching openness on the part of the public to accept those reasons as valid.