Those 10px the [Caption] tag adds to the attachment div, likely breaking your style in the process, are fixed with the following. The style tag setting width has been commented out.
add_filter( 'img_caption_shortcode', 'cleaner_caption', 10, 3 );
function cleaner_caption( $output, $attr, $content ) {
/* We're not worried abut captions in feeds, so just return the output here. */
if ( is_feed() )
return $output;
/* Set up the default arguments. */
$defaults = array(
'id' => '',
'align' => 'alignnone',
'width' => '',
'caption' => ''
);
/* Merge the defaults with user input. */
$attr = shortcode_atts( $defaults, $attr );
/* If the width is less than 1 or there is no caption, return the content wrapped between the < tags. */
if ( 1 > $attr['width'] || empty( $attr['caption'] ) )
return $content;
/* Set up the attributes for the caption <div>. */
$attributes = ( !empty( $attr['id'] ) ? ' id="' . esc_attr( $attr['id'] ) . '"' : '' );
$attributes .= ' class="wp-caption ' . esc_attr( $attr['align'] ) . '"';
/* $attributes .= ' style="width: ' . esc_attr( $attr['width'] ) . 'px"'; */
/* Open the caption <div>. */
$output = '<div' . $attributes .'>';
/* Allow shortcodes for the content the caption was created for. */
$output .= do_shortcode( $content );
/* Append the caption text. */
$output .= '' . $attr['caption'] . '
';
/* Close the caption </div>. */
$output .= '</div>';
/* Return the formatted, clean caption. */
return $output;
}