mask-type property

We discussed at the F2F this week the 'mask-type' property, and we 
decided not to allow it to be specified on arbitrary elements to affect 
how a mask that applies to that element would be interpreted.  (Instead, 
we have "luminance" and "alpha" keywords in the 'mask' property itself.) 
  Instead we decided to make 'mask-type' only apply to <mask>.  This is 
in place of the current maskType="" attribute on <mask>.

I was just implementing this, and I wondered whether it really makes 
sense for it to be a property.  I don't think there are really any use 
cases for using style sheets to change how a <mask> element is 
interpreted.  I think therefore we should stay with a plain attribute on 
<mask>, but perhaps call it something other than maskType="", just to 
avoid the camel-casing issues that Simon brought up.  I think type="" 
would work fine.