I agree with the other commentator in spirit. Just because the spec is free doesn't mean it is suitable for web developers. The HTML5 spec should be written for browser developers.
For example, the C++ spec is available (although, not sure if its free, but imagine it is), but I'd recommend it to VERY few people. I'd much more likely recommend Lippman's text or Koenig's, even if there is a $20 price premium on those books.
In the same way, if this book distills HTML5 into what is necessary and useful for web devs then it is worth $20.
I can't really defend an ebook that isn't even published yet but I don't think your logic tracks.
Current - The book hasn't even shipped yet so I'd assume it will be current for people who purchase it now
Uses Hyperlinks - I love hyperlinks and they're a great feature but I wouldn't call them a deal breaker (and the lack of hyperlinks here is more a limit on the concept of ebooks not a fault of this particular one)
Free - If this book has in fact managed to distill even most of the 900 page spec down to 85 easy to read pages than I think that's more than worth $18. We have web programmers on staff who make double that in an hour.
The HTML5 spec is current, uses hyperlinks and free. Easy choice.