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Not all mobile operators ‘optimise’ images and in my experience even those operators that do only seem to re-compress JPEGs.
For example, in the UK, O2 do re-compress JPEGs but Vodafone don’t even on really low throughput connections.
Even if all operators did re-compress images would you really want to leave the final quality of your images to the operator?
For some operators e.g. O2, you can disable the re-compression of images by adding @no-transform@ to the @cache-control@ HTTP header.
Great post Mr Kadlec!
I’ve got some concerns with eCSSential, as using JS to determine which CSS files should be downloaded interferes with the browsers ability to pre-fetch and prioritise the order resources are downloaded.
Browsers are already making choices on which resources they should download first, for example in this waterfall for enochs.co.uk, the stylesheets that are not applicable are deferred to the end of the page load
http://www.webpagetest.org/result/121205_G8_9079577333c6bffac22fcbc6480e7220/1/details/ (requests 35, 36, 37)
I guess what we in the performance community needs to do is a bit more work on benchmarking approaches like eCSSential vs multiple CSS resources vs a single merged CSS resource.