Note: it would be over-constrained if we specified the core content width as well

<p style="padding-left:50px;padding-right:50px;
             margin-left:100px;margin-right:100px;
             width:400px">It was going . . .</p>

It was going to be one of Rabbit's busy days. As soon as he woke up he felt important,

If the width is over-constrained, right margin setting is not honoured