HTMLとCSSだけで日本国旗と旭日旗を作る方法

<div class="japanFlag"></div>

.japanFlag {
  background-image: radial-gradient(circle, #d7003a 33%, #fff 0%);
  width: 300px;
  aspect-ratio: 3 / 2;
  border: 1px solid;
}

<div class="risingSunFlag"></div>

.risingSunFlag {
  width: 300px;
  aspect-ratio: 3 / 2;
  background:
    radial-gradient(circle at 37%, #d7003a 25%, transparent 25%),
    repeating-conic-gradient(
      from 16deg at 37% 50%,
      #d7003a 0deg 11.25deg,
      white 11.25deg 22.5deg
    );
  background-size: cover;
  background-position: center;
  background-repeat: no-repeat;
  border: 1px solid;
}