Width 100%: Aspect ratio of image

Question

If I add an image to a post, WordPress hardcodes width and height attributes to the img tag.

I added max-width:100% to my CSS, but of course, if my browser window gets smaller, the resized image gets stretched in height.

What can I do about it?

Is there a way to:

A: Solve it with CSS (I would prefer that)?
B: Forbid WP to hardcode width and height?
or
C: Is there a common way of dealing with image widths, I'm not aware of?

Answer 1

A + C:

img.resize-proportionally {
    max-width: 100%;
    height: auto;
}

As an aside, for the above to work, you shouldn't attempt B.
Having width and height attributes in the img tag is good.