Revert #629

[adamwathan]

This PR reverts a change in #629 that removed the following styles from Preflight:

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

That PR was trying to fix an issue where the height: auto declaration was overriding the height attribute on an image in situations like this:

<img src="..." width="250" height="1000">

...but I personally think this default is really useful, and would rather recommend people work around this issue using inline styles if it ever actually affects them:

<img src="..." style="width: 250px; height: 1000px">

[adamwathan]

Closing in favor of #769.