Example of changing allowed html in the events calendar excerpts.

example.php

add_filter('tribe_events_excerpt_allowed_html', 'my_custom_allowed_html' );
function my_custom_allowed_html() {

	// this array contains all the html elements that can be added
	$allowed_html = array(
		'a' => array(
			'class' => array(),
			'href'  => array(),
			'rel'   => array(),
			'title' => array(),
		),
		'abbr' => array(
			'title' => array(),
		),
		'b' => array(),
		'blockquote' => array(
			'cite'  => array(),
		),
		'cite' => array(
			'title' => array(),
		),
		'code' => array(),
		'del' => array(
			'datetime' => array(),
			'title' => array(),
		),
		'dd' => array(),
		'div' => array(
			'class' => array(),
			'title' => array(),
			'style' => array(),
		),
		'dl' => array(),
		'dt' => array(),
		'em' => array(),
		'h1' => array(),
		'h2' => array(),
		'h3' => array(),
		'h4' => array(),
		'h5' => array(),
		'h6' => array(),
		'i' => array(),
		'img' => array(
			'alt'    => array(),
			'class'  => array(),
			'height' => array(),
			'src'    => array(),
			'width'  => array(),
		),
		'li' => array(
			'class' => array(),
		),
		'ol' => array(
			'class' => array(),
		),
		'p' => array(
			'class' => array(),
		),
		'q' => array(
			'cite' => array(),
			'title' => array(),
		),
		'span' => array(
			'class' => array(),
			'title' => array(),
			'style' => array(),
		),
		'strike' => array(),
		'strong' => array(),
		'ul' => array(
			'class' => array(),
		),
	);

	return $allowed_html;
}