In package Markdown when a boolean attribute is used in an embedded html element, the rest of the markdown text is ignored.  The following code demonstrates:

package require tcltest
namespace import tcltest::*
package require Markdown

test booleanattribute {embedded html using a boolean attribute} -body {
  Markdown::convert {
hello

<div allowfullscreen></div>

hello again
  }
} -result {<p>hello</p>

<div allowfullscreen></div>

<p>hello again</p>}