Different from a wedding site, and that is the point
Wedding websites are built over months, on guest lists negotiated twice and dress codes nobody will state plainly. A birthday arrives on a shorter leash. Someone floats a date on Tuesday, the venue is checked for Saturday, and the invitation wants to be in hands by the weekend. The birthday website creator is shaped for that pace: fewer sections to fill, a theme that leads the page, and wording that sounds like a person rather than a stationer.
Casual does not mean thin, though. A good birthday page answers the same four questions every guest asks โ when, where, may I bring someone, is there food โ then adds the part weddings often skip: what the party is actually about. A neon-decade theme, a garden afternoon, a supper where nobody mentions the candles.