When attending a Muslim wedding, a guest should always dress formally and respectfully. Female guests are expected, though not necessarily required, to cover their neck and shoulders, due to the Muslim stance and attitudes regarding feminine modesty.
Female guests are not required to cover their hair with a hijab or scarf at a Muslim wedding, but exposed skin in the upper body area is viewed as immodest and inappropriate. A simple long gown or dress is a good option for a Muslim wedding, though a deep neckline should be covered by a scarf or pashmina of some sort.
Male guests are under no obligation to dress in Islamic-inspired garb, though they are expected to dress formally, as one would when attending a church wedding. A simple, classic suit with a tie is always acceptable.
Muslims are very kind, friendly, and welcoming people who do not make demands of their wedding guests, but they will be deeply honored to see a guest make an effort. Their hospitality and kindness will be particularly evident if a wedding guest takes the time to do research and dress according to Islamic standards, as it shows a high level of respect for their customs and values.