Have you ever come across something that didn’t work well because the person who built it didn’t actually use it? Don’t be that person. Use what you build.
Use it on your computer. Use it on someone else’s computer (preferably one with a different operating system and web browser). Use it on your phone. Use it on someone else’s phone. Use it on a tablet. Walk into an Apple Store and use it on a bunch of different devices.
Don’t just take a quick look. Use it. Actually use it. Find the rough edges. Figure out what’s broken. Take notes. Now prioritize the problems, get to work, and make what you built better for everyone.