<style>
* {
box-sizing: border-box;
}

// ทำให้ทุก element มี  box-sizing: border-box;

#example1 {
width: 300px;
padding: 40px;
border: 15px solid blue;
}

#example2 {
width: 300px;
padding: 10px;
border: 2px solid red;
}
</style>