If an area is optional or empty the user can't click on the area if it deosn't take space. The editor should visualize the place.

The placeholder can be positioned (optional) like this

<div id="main">
  <div cms:placeholder></div>
</div>

Such a placeholder is only rendered if a page is viewed in edit mode

auto injection
if the placeholder is not defined explicitely, then the following algorythm is used to place it:

• if there is a single root div element, it gets injected into it
• if there is no such single root element the placeholder is placed between the area borders