The HTML <div> is [block] in wiki syntax. Blocks are key elements when formatting.

[block(center)]
Some text that will be centered%BR%within the container.
[/block]

[block(background-color:#eee,color:#333,padding:5px 10px)]
Some text.
[/block]

Blocks do not directly accept characters, which has the effect of creating a paragraph surrounding the text inside. Also translates line feeds into <br/> and creates a new paragraph when an empty line is found.

If you need a block that accepts characters use instead [html].

[html(center)]
Some text that will be centered%BR%within the container.
[/html]

[html(background-color:#eee,color:#333,padding:5px 10px)]
Some text.
[/html]