A bit of PHP to preserve html formatting in standard WordPress excerpts

The following is an updated version that appears to work better with WordPress 6.x and Full Site Editing Block themes:

function filter_post_excerpt_attrs( $metadata ) {
    if ( 'core/post-excerpt' === $metadata['name'] ) {
        $metadata["attributes"]["excerptLength"] = false;
    }
    return $metadata;
};

add_filter( 'block_type_metadata', 'filter_post_excerpt_attrs', 10 );

Below is the original code that doesn't seem to work with current WordPress and FSE Block themes - try the above code instead.

/**
* allow formatted text in excerpts
**/
function better_trim_excerpt($text)
{
$raw_excerpt = $text;
if ( '' == $text ) {
$text = get_the_content('');
$text = strip_shortcodes( $text );
$text = apply_filters('the_content', $text);
$text = str_replace(']]>', ']]>', $text);

// Removes any JavaScript in posts (between <script> and </script> tags)
$text = preg_replace('@<script[^>]*?>.*?</script>@si', '', $text);

// Enable formatting in excerpts - Add HTML tags that you want to be parsed in excerpts, default is 55
$text = strip_tags($text, '<strong><b><em><i><a><code><kbd><p><br><ul><ol><li><img>');

// Set custom excerpt length - number of words to be shown in excerpts
$excerpt_length = apply_filters('excerpt_length', 60);

// Modify excerpt more string at the end from [...] to ...
// $excerpt_more = apply_filters('excerpt_more', ' ' . '...');

$words = preg_split("/[\n\r\t ]+/", $text, $excerpt_length + 1, PREG_SPLIT_NO_EMPTY);
if ( count($words) > $excerpt_length ) {
array_pop($words);
$text = implode(' ', $words);

// IMPORTANT! Prevents tags cutoff by excerpt (i.e. unclosed tags) from breaking formatting
$text = force_balance_tags( $text );

$text = $text . $excerpt_more;
} else {
$text = implode(' ', $words);
}
}
return apply_filters('wp_trim_excerpt', $text, $raw_excerpt);
}

// Remove the native excerpt function, and replace it with our improved function
remove_filter('get_the_excerpt', 'wp_trim_excerpt');
add_filter('get_the_excerpt', 'better_trim_excerpt');

I found this code at the bottom of this page: wordpress.stackexchange.com