<div class="wrapper">
  <div class="control">
    <label for="flexBasis">flex-basis:</label>
    <select name="flexBasis" id="flexBasis">
      <option value="auto">auto</option>
      <option value="12vw">12vw</option>
      <option value="20%">20%</option>
      <option value="border-box">border-box</option>
      <option value="content-box">content-box</option>
      <option value="fill">fill</option>
      <option value="max-content">max-content</option>
      <option value="min-content">min-content</option>
      <option value="available">available</option>
      <option value="fit-content">fit-content</option>
    </select>
  </div>
  <div class="container">
    <div class="item">CSS</div>
    <div class="item">CSS is awesome!</div>
    <div class="item">I love CSS!</div>
    <div class="item">CSS is simple, but not easy!</div>
  </div>
  <p>建议使用Firefox Nightly查看,效果更佳!</p>
</div>

@import 'https://fonts.googleapis.com/css?family=Share+Tech+Mono';

@import 'https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css';

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

body {
    width: 100vw;
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    background-image: url('https://s3-us-west-2.amazonaws.com/s.cdpn.io/1376484/jess-harding-lqT6NAmTaiY-unsplash.jpg'),radial-gradient(#28a3dd, #0d7751);
    background-size: cover;
    background-position: center;
    font-family: "Share Tech Mono", monospace;
    font-size: 2rem;
    background-blend-mode: multiply,screen, overlay;
    padding: 2vh;
}
.wrapper {
  display: flex;
  flex-direction: column;
  align-items: center;
}
.container {
    position: relative;
    background-size: cover;
    background-position: center;
    backdrop-filter: blur(20px);
    background-color: rgba(255, 255, 255, 0.5);
    border-radius: 5px;
    box-sizing: border-box;
    box-shadow: 0 1px 3px rgba(0, 0, 0, 0.12), 0 1px 2px rgba(0, 0, 0, 0.24);
    transition: .3s ease;
    margin: 2vw;
    
    display: flex;
    min-width: 80vw;
    min-height: 60vh;
    
}

.control {
  padding-bottom: 5vh;
  margin-bottom: 6vh;
  border-bottom: 3px double currentColor;
  color: #fff;
  text-shadow: 1px 1px 2px rgba(0,0,0,.5);
  font-size: 6vh;
}

.item {
  min-height: 40vh;
  border: 2px solid currentColor;
  border-radius: 4px;
  display: flex;
  justify-content: center;
  align-items: center;
  color: #fff;
  text-shadow: 1px 1px 2px rgba(0,0,0,.5);
  box-shadow: 1px 1px 2px rgba(0,0,0,.5);
  font-size: 6vh;
  position: relative;
}

:root {
  --flex-basis: auto;
}

.item:nth-child(1){
  background-color: #f89;
}
.item:nth-child(2){
  background-color: #90f;
}
.item:nth-child(3){
  background-color: #20c0a9;
}
.item:nth-child(4){
  background-color: #8ac020;
}

.item {
  width: 15vw;
  flex-basis: var(--flex-basis);
}

p {
  font-size: 5vh;
  color: #fff;
   text-shadow: 1px 1px 2px rgba(0,0,0,.5);
}
View Compiled
let selectEle = document.getElementById("flexBasis");
let root = document.documentElement;

selectEle.addEventListener("change", (evt) => {
  root.style.setProperty('--flex-basis', evt.target.value);
});

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.