margin: auto does not work when width not given

(See bug #69852) All images should be left and right equally aligned, e.g. centered. They have the following style attributes:

display: block;
margin: .5em auto .5em auto;

1. Width and height given

<img src="http://bugs.kde.org/kde_logo.jpg" width="296" height="79" alt="me" />
me

2. Only height given

<img src="http://bugs.kde.org/kde_logo.jpg" height="79" alt="me" />
me

3. Only width given

<img src="http://bugs.kde.org/kde_logo.jpg" width="296" alt="me" />
me

4. No width and height given

<img src="http://bugs.kde.org/kde_logo.jpg" alt="me" />
me