For each value in keys in turn, in the order the tokens appeared in the 
attribute's value, run the following substeps

         1. If the value is not a string exactly one Unicode code point
            in length, then skip the remainder of these steps for this
            value.

         2. If the value does not correspond to a key on the system's
            keyboard, then skip the remainder of these steps for this
            value.

         3. If the user agent can find a combination of modifier keys
            that, with the key that corresponds to the value given in the
            attribute, can be used as a shortcut key, then the user agent
            may assign that combination of keys as the element's
            assigned access key and abort these steps.

PROBLEM with subsets 1 and 2: does this, in fact, mean:

         1. If the value is not a string exactly one Unicode code point
            in length, procede to the fallback step for this value.

         2. If the value does not correspond to a key on the system's
            keyboard, then procede to the fallback step for this value.

if so, why not say so?

subset 3 is event more problematic: for that i will submit a separate 
bug