<div class="notice">
  👋 Hey, looks like your browser does not support <code>inset-block-start</code>. Try viewing this demo in Firefox.
</div>
<div class="controls">
  <p>Slide to update the <code>inset-block-start</code> value</p>
  <div class="controls__wrapper">
    <input id="slider" type="range" min="0" max="150" step="1" oninput="sliderChange(this.value)">
  </div>
</div>

<div class="wrapper">
  <div class="box box--inset">
    <output id="output"></output>
  </div>
  <div class="box box--original"></div>
</div>

:root {
  --inset-start: 0;
}

body {
  padding: 1.5rem;
}

.wrapper {
  background: #100e17;
  border-radius: 8px;
  height: 250px;
  padding: 0 15px;
  position: relative;
}

.box {
  align-items: center;
  display: flex;
  justify-content: center;
  height: 100px;
  width: 100px;
}
.box--inset {
  background: #f8a100;
  inset-inline-start: var(--inset-start);
  position: relative;
  writing-mode: vertical-lr;
}

.box--original {
  background: transparent;
  border: 1px dashed #f8a100;
  position: absolute;
  top: 0;
  bottom: 0;
}

.controls {
  background-color: #eaeaea;
  border-radius: 8px;
  display: none;
  grid-template-columns: 100%;
  grid-template-rows: repeat(2, 1fr);
  justify-content: center;
  margin-bottom: 1.5rem;
  padding: 25px 0;
  text-align: center;
  width: 100%;
}

.controls__wrapper {
  margin: 0 auto;
  width: 50%;
}

input {
  margin-right: 15px;
  width: 100%;
}

.notice {
  background: #fff9c4;
  border-radius: 8px;
  color: #827717;
  margin-bottom: 1.5rem;
  padding: 1.5rem;
}

@supports (inset-block-start: 0) {
  .controls {
    display: grid;
  }

  .notice {
    display: none;
  }
}

let root = document.documentElement;

const grid = document.getElementsByClassName("grid");
const slider = document.getElementById("slider");
const output = document.getElementById("output");

function sliderChange(val) {
  root.style.setProperty("--inset-start", val + "px");
  output.innerHTML = val + "px";
}

slider.value = 0;