<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
#area{
border: 1px solid gray;
width: 500px;
height: 100px;
font-weight:
}
</style>
</head>
<body>
<div contenteditable id="area"></div>
font-weight: <button>bold</button>
<script>
document.querySelector('button').addEventListener('mouseup', function(){
document.querySelector('#area').focus();
});
</script>