In Unity, all the contents are built to *.unity3d file. I wonder how web player play this content and present them to us in browser. What are the techniques actually working?
When you build a webplayer it creates a .unity3d file and a .html file
the .html file embeds the .unity3d file correctly so it can be used by webbrowsers.
Kudoo: for an analogy, look at Flash and the Flash Player. As Sycle said above, you embed your content in a web page using object/embed tags, those tell the browser to load our web player which then plays back the content. You can write some simple JavaScript to do web player detection (our default HTML shows you how) and offer an installation prompt if they don’t have it installed yet.