In the demo blog post I linked, it looks like libraw is around 3MB, but that the blogger who wrote that post didn't pay much attention to eliminating cruft and the script is additionally not minified. Minified and compressed it's under 450k.

http://dev.tag.is/rawson.js/ seems to be an emscripten-to-JS compiled dcraw.c, and while less functional than libraw it's under 500kb.

Just tried http://dev.tag.is/rawson.js/ and it doesn't work (it just says "Reading file", the green bar is full, but nothing's happening).

The OP site is quite slow but does work: it produces a JPEG in the end (using Canon 6D original CR2 files).

"LightRoom in the browser" would be the greatest thing ever.

What Lightroom features is the most meaningful for you? What do you think?