MarkDownload is an extension to clip websites and download them into a readable markdown file. However, you must keep in mind that it is not guaranteed to work on all websites. To use this add-on, you simply click the add-on icon while you are browsing the page you want to save offline. A popup will show the rendered markdown so you can make minor edits or copy the text, or you can click the download button to download an .md file. Selecting text will allow you to download just the selected text.
