one can provide a "cascade" of accesskeys for an individual element 
using a space delimited list; to take the example currently in the HTML5 
draft:

QUOTE src-"http://dev.w3.org/html5/spec/editing.html#the-accesskey-attribute"

... the search field is given two possible access keys, "s" and "0" (in 
that order). A user agent on a device with a full keyboard might pick 
Ctrl+Alt+S as the shortcut key, while a user agent on a small device 
with just a numeric keypad might pick just the plain unadorned key 0:

<form action="/search">
<label>Search: <input type="search" name="q" accesskey="s 0"></label>
<input type="submit">
</form>
UNQUOTE

ISSUE 1. cascade order is a very "weak" rather than a strong binding 
-- how does the user know what accesskey to use when multiple accesskeys 
are assigned to an individual element?

ISSUE 2. "limited group of characters" -- there are a very finite 
number of characters that one can use as an accesskey; is the cascade of 
keys set using a space delimited list global?  (that is, does every first 
item listed belong to accesskey-scheme A, the second to 
accesskey-scheme-B, etc.

<form action="/search">
<label>Search: <input type="search" name="q" accesskey="s 0"></label>
<input type="submit" accesskey="= 1">
</form>

in the preceding code sample, is the first accesskey theme:

 1. accesskey for input type="search" = s
 2. accesskey for input type="submit" = =

(for fellow speech users, the first acesskey is the character s while 
the second is the equals-sign)

and the second:

 A. accesskey for input type="search" = 0
 B. accesskey for input type="submit" = 1

(for fellow speech users, the first acesskey is the character 0 while 
the second is the character 1)

SOLUTION: if the above is true, it needs to be explicitly stated in the 
HTML5 spec.