Is it bad practice to direct a user to a mobile friendly side of the site on screen resize

I have an html5 application that consists of two different interfaces. The desktop interface demands a minimum of 900 pixels in width to function properly. The mobile version is built in an entirely different manner to continue the same functionality in a mobile friendly way. Each view receives and uses the same data in the exact same way to make the app easily interchangeable between mobile and desktop. My go-to method is to track the users screen width from page load and before/after screen resizing. If the screen width is smaller than the 900px threshold, the user will be directed to the mobile version of that view they are currently in. Is this process bad practice? Will it hold up as the app becomes more robust and if not, what is a suffice method of handling a desktop/mobile app that cannot utilize css3 responsive design standards.