Just to give you an idea of the complications involved, this is the record of events and Ready State.Completed from navigating (triggered from a second form) to amazon.co.uk

Navigating
Navigated
Navigating
Navigating
Navigated
Doc Completed
Doc Completed
Ready (first setting)
Navigating
Ready
Navigating
Ready
Navigating
Ready
Navigating
Ready
Doc Completed
Ready
Doc Completed
Ready
Doc Completed
Ready
Navigated
Ready
Navigating
Ready
Doc Completed
Ready
Navigating
Ready
Navigating
Ready
Navigating
Ready
Navigating
Ready
Navigating
Ready
Navigated
Ready
Navigated
Ready
Navigated
Ready
Navigating
Ready
Navigated
Ready
Navigating
Ready
Navigated
Ready
Navigated
Ready
Doc Completed
Ready
Doc Completed
Ready
Doc Completed
Ready
Navigating
Ready
Navigated
Ready
Navigated
Ready
Doc Completed
Ready
Doc Completed
Ready
Navigated
Ready
Doc Completed
Ready
Doc Completed
Ready
Doc Completed
Ready
Doc Completed
Ready

So, the conclusion is that you can't really trust the full document being loaded on any event or value of ReadyState. DocumentCompleted will almost certainly be the last event so your best bet if you're looking for a known item in a webpage is to check for it specifically. Otherwise it's all a bit in the lap of the Gods, which is why many prefer to use WebClient to download source HTML for scraping.