I don't know what the rational is for greying them out is, but that's the category.
The consequent highlighted parts reminds me a lot of Chinese, where essentially every word is "important" and there are few filler words, and hence there is often a lot of contextual information.
1. http://nlp.stanford.edu/pubs/CICLing2011-manning-tagging.pdf
More generally, for true syntax highlighting I think you do need the parse tree, and parsers (as opposed to taggers) definitely aren't at 97%.
If you are interested only in the syntactic tag, not in the structure of the tree, the number is somewhat higher.
The 80% figure was quoted from some talk I heard a few years ago, so I concede it's almost certainly improved since then.
Graded as a school exercise, I think 97% wouldn't be that good.
And as POS tagging is usually only done as preprocessing for some other task like syntactically parsing a text (which itself is usually preprocessing for yet another task), 97 % accuracy per word is not as good as it sounds. Parsers need to work with wrong data for every second or third sentence.
(This surprises me. I would expect accuracy for different words in a sentence to be correlated, you either make no errors or several.)
There's basically no part of NLP that's a solved problem. Even something as superficially simple as segmenting running text into sentences and tokens is decidedly non-trivial.
not quite what you're proposing but useful nonetheless
It's really just a way to get people to visit his blog by making a bold statement.