> is encountered in a HTML comment the comment breaks.
Take the following markdown source:
<!-- Here is an example of this working fine. And a second line to show that's not the problem. --> <!-- This isn't visible like you would expect. > Everything from here is visible. -->
Which renders as:
Everything from here is visible. -->
Here is an example of the above part of the question in the preview screen.
There are two comments, one completely hidden the other partially visible.
This means I can't hide some HTML used to get a render. This means if I, or others, need to update the render we either don't have the HTML available or I have to clutter the post.