Currently, it is possible to text link a high quality version of a YouTube video by adding &fmt=18 to the URL, but the code does not work for embedding.
Original Code: href="URL&fmt=18"
Wordpress Correction: href="URL&fmt=18"
Result: Link to high quality video works
Original Code: [youtube=URL&fmt=18]
Wordpress Correction: [youtube=URL&fmt=18]
Result: Embedding of a standard quality video instead of a high quality video
Please consider adding support for embedding high quality YouTube videos.