Picking which sizes to list is pretty straight forward.
#Responsive resize image next to each other download#
As before, the browser may download a lower resolution image due to other factors such as poor connectivity.
The browser uses the first media condition match in sizes, so the order matters. Both of the widths in sizes are in CSS pixels. Each item within sizes is, except for the last entry which is just. If srcset contains a width descriptor, the src is ignored by browsers that support srcset. Each item within srcset is w, eg panda-689.jpg 689w. On a 1x device the browser may download panda-689.jpg, but on a 2x device the browser may download panda-1378.jpg. If the window is 1066px wide or greater, we've signalled that the will be 689px wide. You don't need to specify density, the browser figure that out itself. It can now pick the best resource to load. Via sizes it knows the width of the for a given window width. Via srcset, the browser knows the resources available and their widths. Only for browsers that don’t support srcset Image url Width of the image data Width of the window Width of the img element when the condition matches Fallback width, when no media conditions match The key to understanding this syntax is knowing which of these values refer to the window width, the decoded image width, and the width. That last one is particularly tricky, as images start downloading before CSS is ready, so the width of the cannot be detected from the page layout. The decoded width of each of those image resources. 
In order for the browser to pick the right image, it needs to know: On this blog, content imagery takes up 100% of the article width, but the article isn't always 100% of the window. Images of varying width are commonly used as part of the content on responsive sites.
This is only a hint, even on a 3x device the browser may use the 1x image, perhaps due to poor connectivity. Eg if the 2x resource is picked, it'll be rendered at 50% of the resources width/height 
If you don't specify width/height, the browser will display the image at its native width/height divided by the density.The order of items within the srcset doesn't matter.Each item within srcset is x, eg cat-2x.jpg 2x.This works in all modern browsers, and falls back to src in older browsers.Ī few more rules, not covered in the image above: Fixed size, here or in CSS This is used as the 1x src & by browsers that don’t support srcset Image url Pixel density of screen
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