We started with extension as developers/designers found it especially handy for a quick look-up while working on their projects. Not to worry though - we're working on something for the web as well!
Also, our counterparts got a majority of their traction from browser extensions which made it our obvious priority (even though it wasn't the easiest of options).
Just add a "insert url here" box and do it on your server.
I implicitly distrust anyone who insists on running code on my PC that they could run elsewhere.
Especially.. code they can remotely update