How to resize an image to fit in the browser window?

Question

This seems trivial but after all the research and coding I can\'t get it to work. Conditions are:

1. The browser window size is unknown. So please don\'t propose

Answer 1

For the future generations, if you want a solution that answers 1-6 and does 7 in a way that allows resize beyond to original size, I have developed a complete solution for this problem: