.loading {
  position: relative;
  height: 90vh;
}
.loading:before {
  content: "読み込み中";
  display: block;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, 40px);
}
.loading:after {
  content: "";
  display: block;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%) rotate(0deg);
  width: 50px;
  height: 50px;
  border: 5px solid #000;
  border-right: 5px solid transparent;
  border-radius: 50%;
  animation: loading 2s linear infinite;
}
.loading.is-loaded:before,
.loading.is-loaded:after {
  display: none;
}
@keyframes loading {
  0% {
    opacity: 0.3;
  }
  50% {
    opacity: 0.9;
  }
  100% {
    transform: translate(-50%, -50%) rotate(360deg);
    opacity: 0.3;
  }
}

/* 親要素に基準位置を指定します */
.parent {
  display: flex;
  justify-content: center;
  align-items: center;
  /* ウィンドウ幅を想定して指定していますが適宜変更してください */
  width: 100vw;
  height: 100vh;
  height: 100svh;
}
/* .parentに対して.childを上下左右中央にします */
.child {
  font-size: 2rem;
}