The current way of marking up highlighted keywords in the search snippet is to use
<strong> element, however I would like to use
In D8 there is a lot of talk about shifting to HMTL5, while I am for this, I think we need to better support markup choices per se, so if you want to use HMTL4, XHTML or HTML5 you can, therefore we need to stop doing hard coded markup such as...
$text = preg_replace('/' . $boundary . '(' . implode('|', $keys) . ')' . $boundary . '/iu', '<strong>\0</strong>', $text);
... and make all markup themeable, i.e. this should use theme_mark or similar.