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HTML

              
                <header>
  <div class="control">
    <label for="ending-shape">ending-shape:</label>
    <select name="ending-shape" id="ending-shape">
      <option value="" selected></option>
      <option value="circle">circle</option>
      <option value="ellipse">ellipse</option>
    </select>
  </div>
  <div class="control">
    <label for="size">size:</label>
    <select name="size" id="size">
      <optgroup label="extent-keyword">
        <option value="farthest-corner" selected>farthest-corner</option>
        <option value="farthest-side">farthest-side</option>
        <option value="closest-side">closest-side</option>
        <option value="closest-corner">closest-corner</option>
      </optgroup>
      <optgroup label="length for circle" id="circle">
        <option value="10px">10px</option>
        <option value="10em">10em</option>
        <option value="10rem">10rem</option>
        <option value="10vw">10vw</option>
        <option value="10vh">10vh</option>
      </optgroup>
      <optgroup label="length-percentage for ellips" id="ellipse">
        <option value="10% 20%">10% 20%</option>
        <option value="20% 10%">20% 10%</option>
        <option value="50% 50%">50% 50%</option>
      </optgroup>
    </select>
  </div>
  <div class="control">
    <label for="position">position:</label>
    <select name="position" id="position">
      <optgroup label="bg-position">
        <option value="top">top</option>
        <option value="right">right</option>
        <option value="bottom">bottom</option>
        <option value="left">left</option>
        <option value="center" selected>center</option>
        <option value="top left">top left</option>
        <option value="top center">top center</option>
        <option value="top right">top right</option>
        <option value="center center">center center</option>
        <option value="center right">center right</option>
        <option value="left center">let center</option>
        <option value="bottom left">bottom left</option>
        <option value="bottom center">bottom center</option>
        <option value="bottom right">bottom right</option>
      </optgroup>
      <optgroup label="length-percentage">
        <option value="10px 50px">10px 50px</option>
        <option value="10em 50em">10em 50em</option>
        <option value="10rem 50rem">10rem 50rem</option>
        <option value="10vw 50vw">10vw 50vw</option>
        <option value="10vh 50vh">10vh 50vh</option>
        <option value="10% 50%">10% 50%</option>
        <option value="10%">10%</option>
        <option value="10px 50%">10px 50%</option>
        <option value="10% 50px">10% 50px</option>
      </optgroup>
    </select>
  </div>
</header>

<div class="container">
  <div class="radial__gradient"></div>
  <div class="radial__gradient"></div>
</div>
              
            
!

CSS

              
                @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;
  display: grid;
  grid-template-rows: 100px 1fr;
  font-family: "Exo", Arial, sans-serif;
  background-color: #222;
}

header {
  background-color: rgba(0, 0, 0, 0.85);
  color: #fff;
  display: flex;
  justify-content: center;
  align-items: center;
}

.control {
  margin: 0 5px;
  display: flex;
  align-items: center;
  font-size: 1.2rem;
}

:root {
  --color-start: #ebf;
  --color-center: #09f;
  --color-end: #f36;
  --ending-shape: ;
  --size: farthest-corner;
  --position: center;
}

.container {
  display: grid;
  padding: 10px;
  grid-template-columns: 50vh 1fr;
  gap: 10px;
  place-content: center;
}

.radial__gradient {
  border-radius: 5px;
  min-height: 50vh;
  background-image: radial-gradient(
    var(--ending-shape) var(--size) at var(--position),
    var(--color-start),
    var(--color-center),
    var(--color-end)
  );
}

              
            
!

JS

              
                const selects = document.querySelectorAll("select");
const rootElement = document.documentElement;
const circleOpt = document.getElementById("circle");
const ellipsOpt = document.getElementById("ellipse");

selects.forEach((select) =>
  select.addEventListener("change", (e) => {
    if (e.target.value === "circle" || e.target.value === "") {
      ellipsOpt.setAttribute("disabled", true);
      circleOpt.removeAttribute("disabled");
      rootElement.style.setProperty(`--${e.target.id}`, e.target.value);
    } else if (e.target.value === "ellipse") {
      ellipsOpt.removeAttribute("disabled");
      circleOpt.setAttribute("disabled", true);
      rootElement.style.setProperty(`--${e.target.id}`, e.target.value);
    } else {
      rootElement.style.setProperty(`--${e.target.id}`, e.target.value);
    }
    // rootElement.style.setProperty(`--${e.target.id}`, e.target.value);
  })
);

              
            
!
999px

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