<div class="wrapper">
  <div class="box"></div>
  <div class="box box2"></div>
  <pre class="code"></pre>
</div>

@import url("https://fonts.googleapis.com/css?family=Gochi+Hand");

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  margin: 0;
  padding: 0;
  background-color: #291642;
  font-family: "Gochi Hand", sans-serif;
  color: #fff;
  font-size: 130%;
  letter-spacing: 0.1rem;
  display: flex;
  align-items: center;
  justify-content: center;
  width: 100vw;
  min-height: 100vh;
}

.wrapper {
  width: 50vw;
  height: 50vh;
  min-width: 200px;
  border: 1px dashed #fff;
  overflow: hidden;
  resize: horizontal;
  position: relative;
}

.box {
  min-height: 100px;
  background-color: #f36;
  margin: 2px 0;
}

.box2 {
  width: 100%;
  background-color: hsl(0.5turn, 90%, 30%);
}

.code {
  position: absolute;
  bottom: 10px;
  left: 10px;
  right: 10px;
  text-align: center;
  white-space: nowrap;
}

const wrapper = document.querySelector(".wrapper");
const code = document.querySelector(".code");
const objResizeObserver = new ResizeObserver(function(entries) {
  const entry = entries[0];
  let cr = entry.contentRect;
  const width = cr.width;
  code.textContent = `width: ${width}px`;
});

objResizeObserver.observe(wrapper);