[dom] MutationObservers -- observing certain property changes

Hello,

I'd like to comment on the current spec for MutationObservers.

Some that I have always found confusing is the differentiation between
properties and attributes of DOM Elements. As I am immersed in writing
scripts for the DOM, I usually interact with the DOM via the Node's
properties. Mostly, attributes and properties overlap, however there are
properties that don't mirror their corresponding attributes.

For example, the `checked` property initially has the same value as its
attribute. However, when such an element is checked, the property value
changes whereas the attribute value does not.
As properties are part of the DOM, it would help very much if such
property changes were possible to be observed via MutationObservers.

Kind regards,
Marcel Klehr