宽度40px，另一个的补全宽度：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
html,body,.box{
height:100%;
}
body{
margin:0; }
.box{
display:-webkit-flex;
-webkit-flex-direction: row;
display:flex;
flex-direction:row;
}
.box .child{
width:40px;
background:#f00;
}
.box .child1{
flex:1;
background:#0f0
}
</style>
</head>
<body>
<div class="box">
<div class="child">child</div>
<div class="child1">child1</div>
</div>
</body>
</html>