aspect-ratio

<div id="inner">
  ビューポートの幅と高さを変更しながら、この要素を見ていてください。
</div>

/* 最小アスペクト比 */
@media (min-aspect-ratio: 8/5) {
  div {
    background: #9af; /* blue */
  }
}

/* 最大アスペクト比 */
@media (max-aspect-ratio: 3/2) {
  div {
    background: #9ff; /* cyan */
  }
}

/* 正確なアスペクト比。上書きを防ぐために最下部に配置 */
@media (aspect-ratio: 1/1) {
  div {
    background: #f9a; /* red */
  }
}

<label id="wf" for="w">width:165</label>
<input id="w" name="w" type="range" min="100" max="250" step="5" value="165" />
<label id="hf" for="w">height:165</label>
<input id="h" name="h" type="range" min="100" max="250" step="5" value="165" />

<iframe
  id="outer"
  src="data:text/html,<style> @media (min-aspect-ratio: 8/5) { div { background: %239af; } } @media (max-aspect-ratio: 3/2) { div { background: %239ff; } } @media (aspect-ratio: 1/1) { div { background: %23f9a; } }</style><div id='inner'> Watch this element as you resize your viewport's width and height.</div>">
</iframe>

iframe {
  display: block;
}

outer.style.width = outer.style.height = "165px";

w.onchange = w.oninput = function () {
  outer.style.width = w.value + "px";
  wf.textContent = "width:" + w.value;
};
h.onchange = h.oninput = function () {
  outer.style.height = h.value + "px";
  hf.textContent = "height:" + h.value;
};