<div class="box"></div>

<div class="form">

  <label for="background-position">background-position:</label>
  <select name="background-position" id="background-position">
    <optgroup label="Keyword values">
      <option value="left 100px top 30px">left 100px top 30px</option>
      <option value="left 10% bottom 30vh">left 10% bottom 30vh</option>
      <option value="right 20% bottom 10px">right 20% bottom 10px</option>
      <option value="right 200px top 10%">right 200px top 10%</option>
      <option value="top 10vh left 10%">top 10vh left 10%</option>
      <option value="top 10rem right 200px">top 10rem right 200px</option>
      <option value="bottom 10vh left 20%">bottom 10vh left 20%</option>
      <option value="bottom 10em right 10vh">bottom 10em right 10vh</option>
    </optgroup>

    <optgroup label="Invalid values">
      <option value="left 10px right 10%">left 10px right 10%</option>
      <option value="right 10px left 10vh">right 10px left 10vh</option>
      <option value="top 50px bottom 10rem">top 50px bottom 10rem</option>
      <option value="bottom 10em top 30%">bottom 10em top 30%</option>
      <option value="center 10em right 10%">center 10em right 10%</option>
      <option value="center 10% left 10ch">center 10% left 10ch</option>
      <option value="top 50rem center 20%">top 50rem center 20%</option>
      <option value="bottom 120px center 10rem">bottom 120px center 10rem</option>
      <option value="10px right 20px top">10px right 20px top</option>
    </optgroup>
  </select>

</div>

@import url("https://fonts.googleapis.com/css2?family=Exo:wght@600&display=swap");

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  width: 100vw;
  min-height: 100vh;
  font-family: "Exo", Arial, sans-serif;
  background-color: #557;
  color: #fff;
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 1rem;
  gap: 1rem;
}

.box {
  width: min(100%, 600px);
  background-color: #fff;
  aspect-ratio: 16 / 9;
  box-shadow: 0 0 10px 1px rgb(0 0 0 / 25%);
  border: 1.625em solid rgb(220 20 20 / 38%);
}

:root {
  --position: left 100px top 30px;
}

.box {
  background: #fff
    url("//www.w3cplus.com/sites/default/files/blogs/2022/2207/avatar.svg") left
    top / 100px 100px no-repeat;
  background-position: var(--position);
  overflow: hidden;
  resize: both;
}

.form {
  padding: 1rem;
  background-color: rgb(0 0 0 / 0.6);
  backdrop-filter: blur(20px);
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  gap: 10px;
}

.form select {
  padding: 0.25em 0.5em;
  font-size: inherit;
}

const rootElement = document.documentElement;
const repeatHander = document.getElementById("background-position");

repeatHander.addEventListener("change", (etv) => {
  rootElement.style.setProperty("--position", etv.target.value);
});