Spec section 188.8.131.52, ELResolver Requirements requires the following for isReadOnly():
If running in a non-portlet request or base is non-null, return null.
However the return type of isReadOnly() is boolean so the return value must be either true or false. I recommend that the return value be true since it must be ignored (since ELContext.setPropertyResolved(true) must not be called) according to the documentation of ELResolver) anyway.
So the text should be changed to the following:
If running in a non-portlet request or base is non-null, return true.