XLint does not prevent your code from running, it is simply a tool for locating possible issues. Whether or not the <template> tag will work depends on the target device, the XDK has no say in the behavior of the target device. For Android devices, I recommend you restrict yourself to using Android 4.4+ devices or make sure the "optimize with Crosswalk" is checked if you wish to also include 4.0-4.3 devices. I believe it is support on iOS 9 and 10, but you'll have to build an app and test to confirm. I do not believe it is supported on Windows devices. Ultimately, to be sure, you'll have to build and test on some real devices.