<body>
<div id="left_page">
123123
</div>
<div id="right_page">
dlsjlskd
</div>
</body>
body {
display: flex;
flex-direction: row;
height: 900px;
}
#left_page{
width: 100px;
min-width: 100px;
background: red;
height: 900px;
}
#right_page{
flex-grow: 1;
background: green;
text-align: center;
min-width: 200px;
}