The old way was that the user would install Wolfram Player on his computer and then the browser would use Wolfram Player to show CDFs in the browser. There is a new way that can also show CDFs in the browser but instead of computing the result, the computation happens in the cloud. You can see an example of that here:
Here is a question about this here on Wolfram Community:
I have not tried this but the instructions given by Andre seem straightforward enough to follow.