How do I retain the aspect ratio of an image if its dimensions are percentages?

My images have a 16:9 aspect ratio. I would like each image's wi开发者_开发技巧dth to be 33% of the entire browser window so I can stack 3 images per row:

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When I specify width:33% in CSS, the height does not change to maintain the 16:9 aspect ratio. Apparently, CSS is unlike HTML. In HTML, only one dimension needs to be specified.

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Just add height: auto.. Thus:

img {
     width: 33%;
     height: auto;
}

Then the height adjusts itself according to the width.