The Evernote export function (on my Mac) is offers both an HTML option and their .ENEX option which is XML. Both formats are plain text files with "tags". HTML (HyperText Markup Language) is the language of the "written" Web (web sites, blogs, etc.) while XML (eXtensible Markup Language) is the language of the "data" Web. They aren't proprietary but neither is very easy to read in their raw state. If anything should happen to Evernote, long before they actually shut down, there will be dozens of companies offering easy migration to their platform or service.
We are way too big a market to be ignoresd.