Why HTML WebSocket access Socket.io web socket server failed?

As socket.io said, it supports WebSocket, so, I use HTML5 standard web socket api to access socket.io server, but I always get below error:

WebSocket connection to 'ws://localhost:8080/' failed: Connection closed before receiving a handshake response

Then, I tried to use socket.io client in js to access socket.io server, it works, and by chrome network monitoring, i found it using web socket protocol correctly.

Did anyone ever try W3C Websocket api to access socket.io server and met similar issue? or any ideas or clues for my problem? appreciated!

Test code is here: https://github.com/piginzoo/socketiotest