<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">left 100px top</option>
      <option value="left 10% bottom">left 10% bottom</option>
      <option value="right 20% bottom">right 20% bottom</option>
      <option value="right 200px top">right 200px top</option>
      <option value="top 10vh left">top 10vh left</option>
      <option value="top 10rem right">top 10rem right</option>
      <option value="bottom 10vh left">bottom 10vh left</option>
      <option value="bottom 10em right">bottom 10em right</option>
      <option value="left 15% center">left 15% center</option>
      <option value="center left 20px">center left 20px</option>
      <option value="right 10em center">right 10em center</option>
      <option value="center right 200px">center right 200px</option>
      <option value="top 10vh center">top 10vh center</option>
      <option value="center top 30%">center top 30%</option>
      <option value="bottom 10ch center">bottom 10ch center</option>
      <option value="center bottom 10ch">center bottom 10ch</option>
    </optgroup>

    <optgroup label="Invalid values">
      <option value="left right center">left right center</option>
      <option value="right left top">right left top</option>
      <option value="center 50px left">center 50px left</option>
      <option value="left center 30%">left center 30%</option>
      <option value="10em right top">10em right top</option>
      <option value="10% right 10ch">10% right 10ch</option>
      <option value="top 50rem 20%">top 50rem 20%</option>
      <option value="90px 120px top">90px 120px bottom</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;
}

.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);
});