Did you use a plugin to show excerpts, because Twenty Fourteen does not show excerpts, even if you set them under Settings > Reading. I think an excerpt will be displayed if you use the <!--more--> tag.
One of the dangers of displaying excerpts, as you have discovered, is that if HTML tags are included in the excerpt, you run the risk of closing tags being left off. This is especially noticeable when closing <DIV> tags are left off; it really screws up the appearance of the site. That is why the default behavior of WordPress is to strip off HTML tags before displaying the excerpt. Manual excerpts retain the HTML tag, but it is assumed that when creating a manual excerpt, you will close any opened HTML tags within the excerpt.