<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>rem自适应</title>
<script>
var c = () => {
let w = document.documentElement.clientWidth;
let n = (20 * (w / 320) > 40 ? 40 + "px" : 20 * (w / 320) > 40 + "px");
document.documentElement.style.fontSize = n;
}
window.addEventListener("load", c)
window.addEventListener("resize", c)
</script>
<style>
div {
font-size: 1rem;
}
</style>
</head>
<body>
<div>123</div>
</body>
</html>