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// Remove Height and Width parameters from the_post_thumbnail function
add_filter('post_thumbnail_html', 'remove_thumbnail_dimensions', 10);
add_filter('image_send_to_editor', 'remove_thumbnail_dimensions', 10);
function remove_thumbnail_dimensions($html)
{
    $html = preg_replace('/(width|height)=\"\d*\"\s/', "", $html);
    return $html;
}

/**
 * Disable responsive image support (test!)
 */

// Clean the up the image from wp_get_attachment_image()
add_filter( 'wp_get_attachment_image_attributes', function( $attr )
{
    if( isset( $attr['sizes'] ) )
        unset( $attr['sizes'] );

    if( isset( $attr['srcset'] ) )
        unset( $attr['srcset'] );

    return $attr;

 }, PHP_INT_MAX );

// Override the calculated image sizes
add_filter( 'wp_calculate_image_sizes', '__return_false',  PHP_INT_MAX );

// Override the calculated image sources
add_filter( 'wp_calculate_image_srcset', '__return_false', PHP_INT_MAX );

// Remove the reponsive stuff from the content
remove_filter( 'the_content', 'wp_make_content_images_responsive' );