Browserlink is very simple. The plugin itself hooks autocommands for file changes (and other things) to the provided functions. The functions connect through HTTP to a node.js backend, which your webpage connects also to. The entire process happens extremely fast.
The source for the Node server is here: brolink.js. It uses WebSockets to perform realtime updates, and it looks like it should run on Windows as well.
There are a lot of browser preview scripts for Vim, but they're not usually real-time, so I think Browserlink is worth checking out.