Provide support for image-resizing in-place

[jmarantz]

Current resize_image support is based on HTML width/height tags. However there are no such things in apps. Nevertheless, app authors are in control over the rendered image dimensions and could easily send them along with the URL request if PageSpeed supported a good URL format for that.

Even better, this could be done with client-hints, which would make it standard-compliant.

This is from the mod-pagespeed-discuss thread : https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/mod-pagespeed-discuss/GbntPnwiGTU/c9PX1eHmBQAJ