• Use a browser that supports html5 video: Internet Explorer 9 or higher / any modern browser
    Note: under Windows XP Firefox cannot play mp4 files (Firefox always uses the system video decoder which in XP doesn't support mp4 / H.264)
  • If using your own web server make sure the correct mime type for mp4 is configured: video/mp4
  • Under Windows Server 2008 (R2) there are known problems, see this
  • For VLC in some cases it helps to enable or disable hardware acceleration in Tools - Preferences - Video page
  • If your preferred player is not opening the video files then try to install the following free ffdshow codec