Would there be a way to visualize appereance of a webpage over multiple devices and browser ( at the same time ) without the need to configure complex tests such as selenium ?
Maybe try one of the following:
In Chrome, open DevTools and click on
Toggle device toolbar, then in the dropdown menu:
Edit -> Add custom device...
If it is strictly free online tool you want, then you might be out of luck. None of the services that perform cross browser and multiple devices (Mobile, TV, Tablets and Desktop) testing are free. (Many have either 14 day or 30 days trial)
But if you are willing to pay a little, I'd recommend you to try Browser Stack.
A 30 days trial is available and I'm sure you'll be very impressed by the coverage they provide.
Eye catcher : If yours is an open source project that you want to test, then Browserstack provides free lifetime access to their service. You need to sign up for that and provide the url for your open source project.