Is it possible for Microsoft to make EdgeHTML rendering engine open-source?

Since the announced translation of Edge to the Chromium enigne, articles about such switch harming browsers' ecosystem pop up at least once a week here, on HN. So, I think the step to donate EdgeHTML to open-source community will both address critics' concerns and give less power to Google in long-term. Does such step has any downsides for Microsoft, except some risk of finding security bugs by general public?