The invitation is read on a train, not a desk
Design a wedding page as though guests will open it on a wide screen and you have described a rarer moment than the truth. In reality the reading happens on a phone: in the commute, on a lunch break, on a bus with one thumb, sometimes outdoors in bright sun. If the page only shines on a laptop, it has failed the moment it matters.
A mobile-first wedding page is designed the other way round. The layout is built for a small screen first, so sections stack in a clear order, the type stays legible at arm's length, and the reply is a couple of taps a guest can finish without stopping what they are doing. The wide-screen version is the bonus, not the assumption.