html, body {
    margin: 0;
    padding: 0;
    /* Arial is the choice, sans-serif is the backup plan */
    font-family: Arial, sans-serif;
    background: radial-gradient(circle at top, #ffffff 0, #f0f0f0 45%, #e5e5e5 100%);
    background-repeat: no-repeat;
    background-attachment: fixed;
}

/* . means for class="page-wrapper" */
.page-wrapper {
    padding-top: 40px;
    padding-bottom: 80px;
}

.paper {
    width: 92%;
    max-width: 1200px;
    /* 0 for top and bottom margin, auto set it center horizontally insider parent container */
    margin: 0 auto; 
    background: #ffffff;
    /* round the corner of the container by 3 pixel */
    border-radius: 3px;
    /* 2 light source */
    box-shadow:
        0 18px 45px rgba(0,0,0,0.18),
        0 6px 12px rgba(0,0,0,0.12);
    /* bc the corner is rounded, this make sure that things inside do not stick out */
    overflow: hidden;
    
}

/* !!! part 1 */
.paper-header {
    padding: 15px 25px;
    /* to make this flex container, to enable display: flex; and justify-content: space-between; */
    display: flex;
    /* first element all the way to the left, and last right */
    justify-content: space-between;
    /* vertical center */
    align-items: center;
    /* a line at bottom */
    border-bottom: 1px solid #e0e0e0;
}

/* a means link, so all link in class breadcrumbs */
.breadcrumbs a {
    text-decoration: none;
    color: #333;
    font-size: 20px;
    font-weight: bold;
}

.lang-selector a {
    margin-left: 16px;
    text-decoration: none;
    font-size: 14px;
    color: #666;
}

.lang-selector a.active {
    font-weight: bold;
    color: #000;
}

/* !!! part 2 */
.paper-top-strip {
    background: #e9e9e9;
    border-top: 3px solid #c41717;
    border-bottom: 1px solid #d5d5d5;
    /* 12 for top/bottom; 25 for left/right */
    padding: 12px 25px;
}

.main-nav a {
    display: inline-block;
    margin-right: 40px;
    text-decoration: none;
    font-size: 18px;
    color: #555;
}

/* !!! part 3 */
.paper-content {
    /* 40 for top, 25 for left/right, 60 for bottom */
    padding: 40px 25px 60px;
}

.paper-content h1 {
    margin: 0 0 10px;
    font-size: 32px;
}