css伪元素绘制圆环（安卓移动端椭圆问题）

b::before{
  content: "";
  position: absolute;
  top: 50%;
  left: 0.3rem;
  transform: translate(0,-50%);
  width: 0.7rem;
  height: 0.7rem;
  border: .1rem solid #fff;
  border-radius: 50%;
}

请删除 box-sizing: border-box;