Muslim weddings are loaded with tradition. To be a thoughtful guest, you should be respectful of Muslim traditions and culture. This includes wearing the proper attire to the wedding.
Guidelines for Women
Muslims are conservative when it comes to dress. They do not approve of women showing off their skin in any form. A woman's arms, legs, shoulders, and chest should all be covered. A head covering, such as a scarf, should likely be worn if the wedding takes place in a mosque, as well as long, thick socks to cover the ankles, since women are required to remove their shoes upon entering a mosque. Jewelry is acceptable, and the most appropriate outfit would be a long dress or skirt with long sleeves and high neckline.
Guidelines for Men
Unlike the many restrictions imposed on women in Islamic societies, men are more free to act and wear what they choose. In regards to dress, men are only required to be covered from navel to knee, so they are not nearly as limited as women are. An appropriate outfit for a man to wear to a Muslim wedding would be similar to what he would wear to any other wedding, such as a suit. It's important to note that men must also remove their shoes before entering a mosque and may want to cover their feet accordingly for personal comfort, though it is not necessary.
Other Considerations
Although many Muslim weddings are formal and traditional, there has been a movement towards Westernizing them in recent years. Before attending a Muslim wedding, it is best to ask the bride or groom what type of wedding they plan on having, and if there are any clothing requirements that you should be aware of before you choose an outfit for yourself. If you ask first, you will limit the risk of being embarrassed when you arrive due to inappropriate attire. You do not want to offend your hosts, so checking with them first is the best idea.