Ronald,
I got it now, I think: you are looking for a floating window, right?
There used to be Afloat, but it hasn’t been updated in the last 6 years or so.
I managed to compile it from the source (at Github) but didn’t manage to make the bundle work. (It seems it is supposed to work with EasySIMBL.)
You can also download a precompiled installer from Softonic. But the contained version is even older than the one from Github; furthermore the installer is not correctly code-signed and Softonic is not a trustworthy source, so I didn’t run the installer. Try it at your own risk if you want.
I also found Helium on the App Store. It works like a standalone web browser and it gives you a floating window, indeed.
This could be useful for you, maybe.
There is another app that I’m using from time to time: Screenfloat.
Basically it is only a screenshot tool. If you make a screenshot with Screenfloat the screenshot will automatically float above other windows. The UI of the app is pretty clever: if you hover with the mouse over your floating screenshot it automatically disappears. That is, the floating window is not getting in the way, like it usually happens with floating stuff.
Occasionally the app is surprisingly useful. But, since it only produces floating screenshots, it’s not exactly what you are looking for. But I would give it a try anyway.