<div class="forms">
  <div class="controls">
    <label for="direction">direction:</label>
    <select id="direction">
      <option value="ltr">ltr</option>
      <option value="rtl">rtl</option>
    </select>
  </div>
  <div class="controls">
    <label for="writingmode">writing-mode:</label>
    <select id="writingmode">
      <option value="horizontal-tb">horizontal-tb</option>
      <option value="vertical-lr">vertical-lr</option>
      <option value="vertical-rl">vertical-rl</option>
    </select>
  </div>
</div>

<div id="container">
  <div class="box">
    <div class="item"></div>
    <div class="item"></div>
  </div>
</div>

* {
  box-sizing: border-box;
}
body {
  width: 100vw;
  min-height: 100vh;
  display: flex;
  flex-direction:column;
  align-items: center;
}

.forms {
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 3vh;
}
.controls {
  background-color: rgba(0,0,0,.1);
  padding: 10px;
  border-radius: .5em;
  border: 1px solid rgba(0,0,0,.2);
  margin: 0 10px;
}

.controls select {
  font-size: .9em;
}

.box {
  inline-size: 60vh;
  block-size: 40vh;
  border-radius: 5px;
  border: 1px dashed #f36;
  margin: 10vh;
  position: relative;
}

:root {
  --writingMode: horizontal-tb;
  --direction: ltr;
}

#container {
  writing-mode: var(--writingMode);
  direction: var(--direction);
}

.item {
  width: 50px;
  height: 50px;
  position: absolute;
}

.item:nth-child(1) {
  background: #f9089a;
  inset-inline-start: 20px;
  inset-block-start: 30px;
}

.item:nth-child(2) {
  background-color: #90f90a;
  inset-inline-end: 40px;
  inset-block-end: 60px;
}

const writingMode = document.getElementById("writingmode");
const direction = document.getElementById("direction");
const documentElement = document.documentElement;
writingMode.addEventListener("change", function (evt) {
  documentElement.style.setProperty("--writingMode", evt.target.value);
});

direction.addEventListener("change", function (evt) {
  documentElement.style.setProperty("--direction", evt.target.value);
});