Attributes that do not reflect updates from users

[scottaohara]

As discussed with @spectranaut, as a follow on to #396 it'd be good to add a section to the spec to identify how certain attributes in HTML, such as value, and checked represent default values for their elements, and when users interact with these controls these attributes do not reflect the updated values.

See: https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#concept-fe-dirty