Zopfli is the most efficient DEFLATE compression algorithm, trading run-time performance for file sizes that are typically 3-8% smaller than those produced by zlib with the maximum compression setting. Its output is Deflate-compatible, so no specialized decoder is needed.
We should iterate over all repositories which contain PNG-encoded user interface components, compressing each image with Zopfli and committing the result.
More on Zopfli: http://blog.codinghorror.com/zopfli-optimization-literally-free-bandwidth/