To make cross platform and cross browser website application what is the role of user agent switcher?
Is user agent different thing then rendering engine?
and if some browser use same rendering engine then do we need to check on every browser or one is enough?
But even if you switch the user agent of lets say firefox to safari, it will still use the gecko rendering engine to render it's page.
So to check how a page renders in different browsers you really need to render the page in those different browsers.