mark-down | A Custom Element for rendering Markdown built with Polymer | Web Framework library

After some research online, I found the following nugget about connectedCallback: each time the custom element is appended into a document-connected element. This will happen each time the node is moved, and may happen before the element's contents have been fully parsed.

Therefore, depending on the browser, innerHTML may not in fact be defined when being used. That's why the above snippet, while fine in Firefox, doesn't work in Chrome or Edge.

To solve this problem, place the script tags at the bottom of the body, in which case the element will be parsed first, and the script will know what innerHTML contains.

Another way around this is to wrap the custom element constructor inside a DOM loaded event. That event would look like so: