How to make the max width inherent OR initial (whichever comes first) in css?

Question

I'm trying to have an image auto resize until it fills a div or reaches its initial size. I want it to grow as the div grows but stop when its initial size is reached. I also want it to retain its original aspect ratio the entire time. Is this possible only using css?

Answer 1

Please add some additional detail. If this is as straightforward as you describe, just don't set a width in CSS, and apply a max-width;

img{
   max-width:100%
}

and it won't exceed it's natural width. See fiddle - https://jsfiddle.net/6hLz5em2/