Update: I can confirm this is fixed in 4.16 as per Unreal Engine Issues and Bug Tracker (UE-37710)
Update: I believe this is the same issue here WebBrowser doesn't scale page - UI - Epic Developer Community Forums
In this tutorial by Tesla Dev you can see that the Unreal Engine website fits within the browser viewport: Unreal Engine 4 Tutorial - Web Browser - YouTube
However when I set the browser widget to be 1440x900 it cuts off the right side of the page, which I’m guessing is caused by the browser thinking its viewport is larger, see this screenshot:
Here is a screenshot of what the Unreal Engine website looks like in Chrome resized to 1440x900: