This class adds a 4px border below an image that does not touch the image. This class is not as useful in combination with other image classes.
Because of the way Drupal works, we built this style to only be applied to wrapper tags around image tags. These classes will not work if you apply them to the image tag itself, but can work at any level of wrapper div around the image. For example at the block level you can apply the border class once, and all images in the block will get that style.
<p class="border-underline"> <img alt="waterfall" src="waterfall.jpg" /> </p>