All of the browsers are on board to ship Web Components and have been actively engaged in the standards discussions.
Safari just shipped Shadow DOM v1 in Safari Technology Preview. https://webkit.org/blog/6017/introducing-safari-technology-p...
Edge are actively working on Shadow DOM and Custom Elements support https://blogs.windows.com/msedgedev/2015/07/15/microsoft-edg...
Firefox is working on Shadow DOM and Custom Element support as well.
The odds are good that other browsers will begin landing these features in their stable releases in 2016.