How to auto-resize an image while maintaining aspect ratio

Question

How do you auto-resize a large image so that it will fit into a smaller width div container whilst maintaining its width:height ratio?

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Example: stackoverflow.com - when an image is inserted onto the editor panel and the image is too large to fit onto the page, the image is automatically resized.

Answer 1

Do not apply an explicit width or height to the image tag. Instead, give it:

max-width:100%;
max-height:100%;

Also, height: auto; if you want to specify a width only.

Example: http://jsfiddle.net/xwrvxser/1/

img {
    max-width: 100%;
    max-height: 100%;
}

.portrait {
    height: 80px;
    width: 30px;
}

.landscape {
    height: 30px;
    width: 80px;
}

.square {
    height: 75px;
    width: 75px;
}

Portrait Div
<div class="portrait">
    <img src="https://i.sstatic.net/xkF9Q.jpg">
</div>

Landscape Div
<div class="landscape">
    <img src="https://i.sstatic.net/xkF9Q.jpg">
</div>

Square Div
<div class="square">
    <img src="https://i.sstatic.net/xkF9Q.jpg">
</div>