There are 3 different markdown components that look promising
marked-element was created asa part of the Polymer elements and was the first markdown parsing element that I saw. It looks promising except for one thing: I haven't figured out how to use highlight.js with it. This renders the tool less than ideal for the kind of writing I want to do.
juicy-markdown presents the best selection of markdown elements. It allows you to use Github Flavored Markdown but it forces you to use the download of a scrapped version of the Github CSS file. This doesn't always work.
webcomponents.info has a selection of Markdown viewer and editor components. re-markable and juicy-markdown were selected from that list.