It is not reasonable to italicize just _part_ of a word, especially when you're dealing with code and names that often appear with multiple underscores. Therefore, GFM ignores multiple underscores in words:
Image tags with a video extension are automatically converted to a video player.
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@@ -780,7 +780,7 @@ A link starting with a `/` is relative to the wiki root.
- The [Markdown Syntax Guide](https://daringfireball.net/projects/markdown/syntax) at Daring Fireball is an excellent resource for a detailed explanation of standard markdown.
-[Dillinger.io](http://dillinger.io) is a handy tool for testing standard markdown.