It does not even pass the ACID2 test. It is however the best independently developed browser engine out there. If you want an alternative to Webkit and whatever Firefox uses, Netsurf is the only viable option at the moment.
It's still relatively big, because web standards are incredibly complicated, but it's almost tractable.
If you care about browser diversity, it's important.
https://www.ekioh.com/flow-browser/ is fully independent and can run Gmail, but it's commercial / non-FOSS. It's funded by contracts in the set-top-box industry.
https://sciter.com/ is non-FOSS with a free edition but you shouldn't run untrusted javascript on it
And there are some lightly-maintained forks of the leaked Presto 12.x engine from before Opera became a Chromium derivative, but of course it's copyright infringement.
Flow browser passes ACID3 [1], closed source but independent (afaik, dont't really know much about it)
Previous HN discussion: https://news.ycombinator.com/item?id=23508979