A standing wave viewed from another reference frame
An electromagnetic standing wave is the linear superposition of two plane waves of similar amplitude and frequency, travelling in opposite directions. However, if an observer, travelling with uniform velocity with respect to the rest frame, observes the radiation they see something completely different. One wave is boosted and blue-shifted, the other is inverse boosted and redshifted. The combination is not a standing wave.
The simulation shows the two waves and their resultant (black). A slider allows the adjustment of v/c to be between 0 and 0.99. The simulation can be animated by right clicking on the time slider or clicking the play button at the bottom left. The conclusion is that electromagnetic standing waves are a frame-dependent phenomenon!