Thais are devoted to the royal family and show utmost respect to the king, the queen, and their children. While they are generally tolerant of foreigners breaking cultural taboos, no such leniency is shown for disrespecting the monarchy. This includes acts that might seem inconsequential in the West, such as not standing to attention when the national anthem is played in cinemas.
When visiting a temple in Thailand, do not wear shorts or sleeveless shirts. In the case of Wat Phra Kaeo, this also includes sandals. People who are inappropriately dressed need to rent clothes to cover the offending parts or they are refused entry.