Modern servers and browsers support the DEFLATE algorithm for compressing over HTTP, which is the same algorithm as PNG. The only difference is that PNG preprocesses with a predictor to exploit row-to-row correlations in an image. Since the assumptions of that predictor are less likely to be sound with JavaScript code than with practical image data, and since lossless compression is a zero-sum game, PNG will perform worse than plain old DEFLATE on average for JS code.

Maybe, but obviously we would need loss-less compression. And the current web standards like gzip are optimized and perfectly suited for "text-like" loss-less compression, so it seems a little pointless to apply any image-optimized loss-less compression.