<div class="notice">
  👋 Hey, looks like your browser does not support <code>inset-block</code>. Try viewing this demo in Firefox.
</div>

<div class="wrapper">
  <div class="box box--inset"><code>15px 45%</code></div>
  <div class="box box--original"></div>
</div>
:root {
  --start: 15%;
  --end: 45%;
}

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-block: var(--start) var(--end);
  position: relative;
  writing-mode: horizontal-lr;
}

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

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

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

  .notice {
    display: none;
  }
}

External CSS

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External JavaScript

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