Here if anyone has questions on the implementation or such.
1. Scaling down in linear colorspace is essential. One example is [1], where [2] is sRGB and [3] is linear. There are some canary images too [4].
2. Plain bicubic filtering is not good anymore. EWA (Elliptical Weighted Averaging) filtering by Nicolas Robidoux produces much better results [5].
3. Using default JPEG quantization tables at quality 75 is not good anymore. That's what people referring as horrible compression. MozJPEG [6] is a much better alternative. With edge detection and quality assessment, it's even better.
4. You have to realize that 8-bit wide-gamut photographs will show noticeable banding on sRGB devices. Here's my attempt [7] to reveal the issue using sRGB as a wider gamut colorspace.
[1] https://unsplash.com/photos/UyUvM0xcqMA
[2] https://cloud.githubusercontent.com/assets/107935/13997633/a...
[3] https://cloud.githubusercontent.com/assets/107935/13997660/b...
[4] https://cloud.githubusercontent.com/assets/72159/11488537/3d...
[5] http://www.imagemagick.org/Usage/filter/nicolas/