I wrote a Markdown extension which provides tags that define basic layout. Your blog's template ultimately defines the layout. Most templates include layout.css which provides these features.

{<<} This line sits in the left margin of the page

You can including multiple lines inside the same layout tag by indenting them.

The tag… should make its contents… by applying this CSS class…
{<<} extend into the left margin wide left
{>>} extend into the right margin wide right
{<>} extend into both margins wide
{>} float right inside the page right inside
{<} float left inside the page left inside
{|<} hang in the right margin right margin
{>|} hang in the left margin left margin
{||} sit inside a 1/2 width column two column
{|||} sit inside a 1/3 width column three column
{||||} sit inside a 1/4 width column four column