h264 or mp4 with mediaelement.js

I've been using the mediaelement.js(http://mediaelementjs.com/) for our site. I've noticed that I have to convert all the mp4 videos with the codec h264 . If I use the code mpeg-4 for my mp4 video the audio is ok but the video 开发者_开发问答is black.

any idea why is that ?

thanks.

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MEJS requires h264 for .mp4 formatted files.