<!DOCTYPE html>
<html>
<head>
<style>
#myDIV {
width: 50%;
background-color: lightblue;
}
</style>
</head>
<body>
<p>Click the "Try it" button to make sure that the DIV element's width never is below 400 pixels:</p>
<button onclick="myFunction()">Try it</button>
<div id="myDIV">
<p>This DIV element has a defined width of 50%.</p>
<p>The width of this DIV element is 50% of it's parent element.</p>
<p>Try resizing the browser window to see that width of this DIV element will grow/shrink along width the resizing.</p>
<p>But if you click the "Try it" button, yo will make sure that this DIV element does not get below 400 pixels in width.</p>
</div>
<script>
function myFunction() {
document.getElementById("myDIV").style.minWidth = "400px";
}
</script>
</body>
</html>