JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <!-- Bad double-applied fractions in Chrome:
       items size to 40% (80px) and 50% (100px) of their max-content size (200px each),
       so the grid container is 180px wide,
       then they fill 40% and 50% of that space,
       smaller than their original requested size.
  -->
  <div class="grid" style="grid-template-columns: 0.4fr 0.5fr;">
    <div class="grid-item"><div class="inlineBlock"></div><div class="inlineBlock"></div></div>
    <div class="grid-item"><div class="inlineBlock"></div><div class="inlineBlock"></div></div>
  </div>

  <!-- Good fractions in Chrome:
       items size to 50% (100px) and 100% (200px) of their max-content size (200px each)
       so the grid container is 300px wide,
       then they fill 1/3 and 2/3 of that space,
       exactly their original requested size.
  -->
  <div class="grid" style="grid-template-columns: 0.5fr 1fr;">
    <div class="grid-item"><div class="inlineBlock"></div><div class="inlineBlock"></div></div>
    <div class="grid-item"><div class="inlineBlock"></div><div class="inlineBlock"></div></div>
  </div>
</div>

<!-- Same examples, but translated to Flexbox now. -->
<div class="container">
  <div class="flex">
    <div class="flex-item" style="flex: .4;"><div class="inlineBlock"></div><div class="inlineBlock"></div></div>
    <div class="flex-item" style="flex: .5;"><div class="inlineBlock"></div><div class="inlineBlock"></div></div>
  </div>
  <div class="flex">
    <div class="flex-item" style="flex: .5;"><div class="inlineBlock"></div><div class="inlineBlock"></div></div>
    <div class="flex-item" style="flex: 1;"><div class="inlineBlock"></div><div class="inlineBlock"></div></div>
  </div>
</div>

CSS

.grid-item {
  min-width: 0px;
}

.grid {
  display: grid;
  background: orange;
  border:5px solid red;
  float: left;
  clear: both;
}

.flex {
  display: flex;
  background: orange;
  border: 5px solid red;
  float: left;
  clear: both;
}

.inlineBlock {
  display:inline-block;
  width: 100px;
  box-sizing: border-box;
  background:gray;
  height:20px;
  border:1px solid black;
}

.container {
  width:500px;
  height:200px; 
  border:1px solid black;
  background-image: repeating-linear-gradient(to right, white 0px, white 99px, silver 99px, silver 100px);
}

.grid > :nth-child(1), .flex > :nth-child(1) {background: lightblue;}
.grid > :nth-child(2), .flex > :nth-child(2) {background: yellow;}