No. If a spec doesn't define a behavior code can jump to some "undefined" handler which could be anything from a no-op to some quirks mode. Unless you're Microsoft writing specs "do what Word 97 does", copying the behavior of existing browsers is not a specification.
Please don't ignore the context. We are talking about Web browsers.
You don't, in reality, have the latitude to do "anything from a no-op to some quirks mode" of your choice. The requirement is absolutely the one stated: to be compatible with what other browsers are doing. If your browser doesn't satisfy that requirement, then you break the Web, regardless of whether the spec is a hundred words or a hundred million. No amount of pointing at a standard and arguing that it doesn't specify clearly defined behavior in some area will ever be enough to teach a site to be able to say, "Oh, I'll just unbreak myself then so you can go ahead and view/use this page on your computer."
Besides that, even if you were right—and to be clear, you aren't—that doesn't change the fact that, again, arguing for underspecification because "a couple defined values" isn't as much "actual code" that "still needs to be written" is an argument that approaches a problem that isn't I/O bound as if it is.
