This is what is needed for what liquid images are commonly called:
img { max-width: 100%; height: auto; }
The first declaration ensures that all images do not exceed the width of their containing element. An automatic declaration for height ensures that all images retain their proportions when reduced, even if they have size attributes in the img element.
That way you can simply declare the width or maximum width in the img container element without worrying about size / proportions.
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