tools to automate UI testing of a website in multiple devices [on hold]

I know that asking for a tool recommendation is not a very good question in SO. However I saw that there were many previous questions asking the very same thing about other technologies such as testing Java swing apps, iphone testing etc which received very good responses, so I hope I will get a few too.

My problem statement is this: We have a mobile app built for android and iOs (two versions using native code). This works perfectly fine. Now we are converting a portion of our website to be mobile compatible and more importantly, have the same look and feel as the mobile app. This is built using standard HTML5/CSS3/Bootstrap.

We want to automate testing this UI across multiple devices. Are there any standalone/online tools or services that you guys know about that can help with this? I have looked at Xamarin and Selenium Web Driver. Those are good, however I wonder if anyone here knows a better option?

Any suggestions are very much appreciated!