Good day. I need to distract myself from trying to reanimate a Windows-PC. Quite frustrating and pretty unsuccessful so far, but let's see. Why a certification of the abilities of a web-developer? Well, there are plenty of good reasons. Let's get into them.
First of all, everyone doing something seriously should be able to get a certificate on his or her field of work. If you're a professional, why bother taking a test and tell the world you made it? I'm getting your point here: a test only for the sake of a test isn't worth it, right.
Web standards are a very serious matter. Of course, the average internet user doesn't bother if a page doesn't look cool or something, even flash-based pages impress. But when it comes to accessability, typography, usability, web standards try to give developers a good set of tools one can use to improve the overall experience of both the developer and the consumer. There is no direct impact of whether you're still using tables or already using divs to layout your content, but in the long run it will pay out, even if it's only that you made it to decrease your traffic costs.
Another very important reason is that web-development and web-design is still terribly underpaid. Many of my friends are doing such things for .. almost nothing, while some well-established companies are building terrible html4 pages and getting a huge load of money for it. And that's where a certification would kick in. And by proving customers that you are indeed worth it, everyone with the real ability to build web-based stuff would certainly have the realistic chance to increase his or her income.
In my opinion, this certification should be something free, a community should build it, I don't know how but I'd be glad to have such a thing.