Margins collapsing through empty elements

Simple tests

There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.

Moderate tests

There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.
There should be a 1em gap between this...
...and this.