方案一：利用padding-left预留空间，左侧脱标，右侧宽度相当于父元素的100%
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>左侧宽度固定，右侧宽度自适应</title>
<style>
.container{
padding-left: 300px;
100%;
height: 500px;
background-color: cadetblue;
position: relative;
}
.left{
position: absolute;
left: 0;
top: 0;
300px;
height: 500px;
background-color: brown;
}
.right{
height: 500px;
100%;
background-color: aqua;
}
</style>
</head>
<body>
<div class="container">
<div class="left">left</div>
<div class="right">
right
right
right
right
right
</div>
</div>
</body>
</html>
方案二：左侧设置浮动，右侧不指定宽度
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
.left{
float: left;
500px;
height: 500px;
background-color: cadetblue;
}
.right{
height: 500px;
background-color: brown;
}
</style>
</head>
<body>
<div class="left">left</div>
<div class="right">right</div>
</body>
</html>