I was surprised years ago (and somewhat still am) with how low the bar got set on what was an acceptable UX on a website. Much of that would have NEVER been tolerated in a desktop app.
A lot of that was due to the previously mentioned mess that was and is browser support. Things are a lot, lot better now in this respect with the ubiquity of Bootstrap, JQuery and the like. You can be reasonably confident how something will look across different screen sizes using the developer tools in the various browsers without actually needing physical devices.