Seamless Animated RGB Rainbow strip - https://codepen.io/richarddewit/pen/VwjEWEL

rainbow.css

.rainbow-wrapper {
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  height: 10px;
  width: 100%;
  overflow: hidden;
}

.rainbow {
  position: absolute;
  left: 0;
  top: 0;
  height: 100%;
  width: 200%;
  background: linear-gradient(
    90deg,
    #ff2400,
    #e81d1d,
    #e8b71d,
    #e3e81d,
    #1de840,
    #1ddde8,
    #2b1de8,
    #dd00f3,
    #dd00f3,
    #ff2400,
    #e81d1d,
    #e8b71d,
    #e3e81d,
    #1de840,
    #1ddde8,
    #2b1de8,
    #dd00f3,
    #dd00f3,
    #ff2400
  );
  transform: translate3d(0, 0, 0);
  animation: rainbow 6s linear infinite;
}

@keyframes rainbow {
  100% {
    transform: translateX(-50%);
  }
}

rainbow.html

<div class="rainbow-wrapper">
   <div class="rainbow"></div>
</div>