must/should/may

Clearly, I was confused on this one.  Ian's mail said,

"* use the WAI style of the imperative voice for
guidelines and checkpoints."

(I remembered about guidelines but not checkpoints.)

Daniel's message ([1]) said essentially the same (or alternately, use 
priorities or normative keywords, not both).

Sorry if I created confusion here!

-Lofton.

[1] http://lists.w3.org/Archives/Public/www-qa/2002Jan/0011.html

Excerpt from WCAG Checklist...

Priorities
Each checkpoint has a priority level assigned by the Working Group based on 
the checkpoint's impact on accessibility.
[Priority 1]
A Web content developer must satisfy this checkpoint. Otherwise, one or 
more groups will find it impossible to access information in the document. 
Satisfying this checkpoint is a basic requirement for some groups to be 
able to use Web documents.
[Priority 2]
A Web content developer should satisfy this checkpoint. Otherwise, one or 
more groups will find it difficult to access information in the document. 
Satisfying this checkpoint will remove significant barriers to accessing 
Web documents.
[Priority 3]
A Web content developer may address this checkpoint. Otherwise, one or more 
groups will find it somewhat difficult to access information in the 
document. Satisfying this checkpoint will improve access to Web documents.