WordPress editor can be really annoying, you probably experienced that. You can add a small function to your function.php file and voilà!
// STOP WORDPRESS REMOVING TAGS
function tags_tinymce_fix( $init )
{
// html elements being stripped
$init['extended_valid_elements'] = 'div[*],article[*]';
// don't remove line breaks
$init['remove_linebreaks'] = false;
// convert newline characters to BR
$init['convert_newlines_to_brs'] = true;
// don't remove redundant BR
$init['remove_redundant_brs'] = false;
// pass back to wordpress
return $init;
}
add_filter('tiny_mce_before_init', 'tags_tinymce_fix');