Seems like Io's "forward" is a lot like "did-not-understand" (and similar mechanisms in Ruby/etc. languages), but it takes priority over messages that would've otherwise been understood.

Yes its same as did-not-understand/method_missing/__getattr__/AUTOLOAD but no it doesn't take priority over other defined messages.

For eg, If I change the first line in my code above to:

    Builder := Object do (
        foo := method (arg, write (arg))
    )

And do:

    Builder do (
        h1 (
            foo ("Foo here!")
            p ("paragraph")
        )
    )

I get a untagged "Foo Here!":

    <h1>Foo here!<p>paragraph</p></h1>