* {
    margin: 0;
    padding: 0;
    box-sizing: content-box;
}

body {
    font-family: Arial;
    background: #f2f2f2;
    padding: 30px;
}

h2 {
    margin-bottom: 20px;
}

.demo-block {
    background: #cfe8d5;
    margin-bottom: 20px;
}

.block1 {
    width: 200px;
    border: 2px solid black;
    margin: 10px;
    padding: 0;
}

.block2 {
    width: 200px;
    border: 10px solid red;
    margin: 20px;
    padding: 20px;
}

.block3 {
    width: 300px;
    height: 150px;
    border: 3px solid green;
    margin: 30px;
    padding: 10px;
}