One of the first questions historically asked when developing a web-based application is What desktop browsers and versions shall we support? These days, you should not be thinking so much about the software installed on the laptops and desktops of your users, but rather the devices they own.
The answer might still likely include a laptop or desktop, though no longer is it a safe assumption that these are the main devices your users use to interact with your application. Mobile phones, tablets, and even wearable devices such as watches are becoming more popular. Depending on your target market, this might be larger devices such as cars or a vending machine! So, make sure that you think big and understand all the types of devices your customers and their customers might want to interact with your application.