Markdown / HTML exporter and hidden blocks

[YousefED]

Perhaps the "default" exporter should skip hidden / invisible blocks:

          Shouldn't this be a paragraph with "Add image"? I guess it's probably because the markdown conversion prioritizes the `img` element even if it has `display: none`. Worth changing to unmount instead of using `display: none`?

Originally posted by @matthewlipski in #428 (comment)