Q. how do I cite youtube videos in MLA formal?

According to the Purdue OWL (which is based on the MLA Handbook) there is no specific "MLA YouTube" format. See the Purdue OWL link, below.

Based on MLA standards for other media formats, OWL suggests that the following format is the most acceptable for citing YouTube videos:

Author's Name or Poster's Username. "Title of Image or Video." Media Type Text. Name of Website. Name of Website's Publisher, date of posting. Medium. date retrieved.

Here is an example of what that looks like:

Shimabukuro, Jake. "Ukulele Weeps by Jake Shimabukuro" Online video clip. YouTube. YouTube, 22 Apr. 2006. Web. 9 Sept. 2010.