User blog comment:MisterWoodhouse/The first migrations to the Unified Community Platform/@comment-25999669-20201117191606/@comment-1869454-20201119024344

Nerdtastic1221, your screenshot shows quite clearly that you're using the Visual Editor in Source mode - not the Source Editor. Yes, this affects the way Previews work, and you'll have a better time of things if you change your Editing settings through your preferences panel.

Either way, though, you also need to remember that to some extent page layouts depend on the browser used to view them. A user with a smaller or larger display than your own isn't going to get precisely the same layout as you do, full stop.