Help me in solving HTMLPR10 problem

help
,
1

My issue

i have doubt in this

My code

header {
    background-color: #333;
    color: white;
    padding: 10px;
}

body {
    font-family: Avenir, sans-serif;
    margin: 0;
    padding: 0;
}

main {
    max-width: 800px;
    margin: 0;
    padding: 20px;
}

img {
    width: 100%;
    height: auto;
}

Learning course: Learn HTML / CSS
Problem Link: Task 5 in HTML / CSS

2

What is your doubt