From the shadow DOM spec author: https://groups.google.com/a/chromium.org/forum/...

bzbarsky on Feb 5, 2014 | parent | context | favorite | on: “The working group should not agree to freeze what...

From the shadow DOM spec author: https://groups.google.com/a/chromium.org/forum/#!msg/blink-d...

There's been a lack of progress in that it's taking a while, but there are a bunch of still-open unresolved issues... The problem with big complex features. ;)