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background: url(image.jpg) fixed 50%; background-size: cover;
fixed (background-attachment) is optional.
The above is simply a shorthand for:
background-image : url(image.jpg); background-attachment : fixed; background-position : 50% 50%; background-size : cover;
You can use in all modern browsers , but Safari :
background: url(image.jpg) fixed 50% / cover;
where / used in background reduction to separate and distinguish position from size (because the position accepts both single and multiple (X, Y) values), but Safari has an error with this / abbreviated use as described above.
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