There is a box style that can be used with a <div> to surround text in a box.

<div class="box">This stuff is in a box</div>

Produces:

Can be combined with icons:

<div class="box"><<warning>> Don't run with scissors.</div>

Produces:

Combining HTML and Markdown can easily break the Markdown renderer --- you may need to resort to full HTML if you want to do anything fancy inside the box.

For example:

<div class="box"><<warning>> **Don't** run with scissors.</div>

Will produce the following incorrect output:

Wheareas the following will work:

<div class="box"><<warning>> <strong>Don't</strong> run with scissors.</div>