The HTMLClass.__getitem__ method (used to get a property of a
class) honors values set in the current form.  This is used so that
when you submit a form, get an error, and redisplay the current
page, the submitted values are "sticky".  But, this support is not
in HTMLItem.  So, it works only for the "primary" object associated
with a form.  Moving this logic into HTMLProperty makes this work
for everything on the form.