JSFiddle - React, Tailwind, and code Playground

HTML

<div class='a'>
    <div class='b'>70%</div>
</div>

CSS

.a{
    width : 90%;
    height : 30px;
    background : blue;
    margin-left:auto;
    margin-right:auto;
}
.b{
    width : 40%;/*比例*/
    height : 100%;
    background : red;
    float:left;
}