Tapestry 4 and earlier had the "$content$" marker that would limit the content in the template. A new T5 element, <t:content>, would accomplish much the same thing. It would allow the outermost portion of a page template to be "placeholder" (for WYSIWYG), with the central part of the template marked as the <t:content>.
<t:content> would not be allowed to nest.
The Tapestry 4 $remove$ marker has already been fixed as