Well it's pure JS in that it's been running the C tesseract through emscripten. So in a way it's pure JS just as much as the original lib is pure assembly when compiled ;-)
As another commenter mentioned, Tesseract.js won't perform very well on 'natural' images (e.g. the very light text you tried).
It should work better if you feed it a screenshot of the black text at the top of the demo page though (Tesseract.js is a pure Javascript port etc...).
> Dropan Enghsh Wage on (Ms page to OCR m
Should be
> Drop an English image on this page to OCR it!