Responsive Image full screen and centered - maintain aspect ratio, not exceed window

Question

So I want an img to be displayed

• as big as possible (filling the width when it is landscape / height when it is portrait)
• no crop
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Answer 1

yourimg {
position: fixed;
left: 0;
top: 0;
width: 100%;
height: 100%;
}

and make sure there is no parent tags with position: relative in it