gistfile1.php

function sm_custom_html_tags() {
	global $allowedposttags;

	$allowedposttags['iframe'] = array(
		'src'                   => true,
		'width'                 => true,
		'height'                => true,
		'frameborder'           => true,
		'webkitallowfullscreen' => true,
		'mozallowfullscreen'    => true,
		'allowfullscreen'       => true,
	);
}
add_action( 'init', 'sm_custom_html_tags', 10 );