Css units

<div class="box"></div>

body{
    width: 300px;
}

.box {
    /* ⚜️ 50% of the width of body (300px) */
    width:50%;
    height: 100px;
    background-color:gold;
    border-top: 3px solid orange;
}

body{
    margin: 0px;
}

.box {
    /* ⚜️ 50% of the viewport */
    width:50vw;
    /* ⚜️ 100% of the viewport */
    height: 100vh;
    background-color:gold;
    border-top: 3px solid orange;
}

body{
    margin: 0px;
}


.box {
    /* ⚜️ because the font size defaults is 16 px
    so width will be 10 * 16 = 160 px */
    width:10em;
    height: 100vh;
    background-color:gold;
    border-top: 3px solid orange;
}

body{
    margin: 0px;
}

.box {
    /* ⚜️ change font size */
    font-size:20px;
    /* ⚜️ because the font size defaults is 16 px
    so width will be 10 * 20 = 200 px */
    width:10em;
    height: 100vh;
    background-color:gold;
    border-top: 3px solid orange;
}

استفاده از
rem
محاسبه رو آسون تر می کنه

html{
    /* 1️⃣ it means 62.5% of 16px = 10px 
    so 10px will be root font size!*/
    font-size:62.5%;
}

body{
    margin: 0px;
}

.box {
    /* 2️⃣ rem x root font size 
    the result will be 150 px*/
    width:15rem;
    height: 100vh;
    background-color:gold;
    border-top: 3px solid orange;
}