Unable to play embedded MP4 videos

[bazart]

I'm using Xara 11 premium on Windows 7 64 bit. The video appears to load OK in design mode, but won't play and is replaced by a black cell when testing.

Judging from the number posts regarding this issue it appears to be a fault with the design of Xara? If anyone can suggest a solution I would be most grateful.

[siran]

Are you sure your mp4 is comatible to the browser you tested in?
It could be worth to explicitly convert it and retry.
Example conversion with ffmpeg

ffmpeg -i in.mp4 -vcodec libx264 -movflags faststart -b:v 800k -an -vf scale=1040:-1 -map 0 out.mp4

where in.mp4 is your source file, out.mp4 is the name of the file to be created, 800k defines the video bitrate and 1040 is the picel width the target video should be created with.
To also include audio you should replace -an with

-acodec aac -ac 2 -ar 48000 -b:a 96k

where 2 means stereo, 48000 is the sampling frequency of the audio to be created and 96k is the bitrate.