Move to a new line using CSS not <br />

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I've been modifying the search widget for a decade to move the button from inline to the next. Having a bit of spare head space I wondered if it could be done with css. The solution was to use the existing class for the button and add [display:block]. There was then the extra positioning using the subsequent three lines.

CSS div.compact_search_form input.search_submit{
  color: blue;
  font-weight: bold;
  margin:0.6em auto;