How to make it play movie.mp4 on the browser?

<video src="http://myDomain.com/myMovie01.MOV" controls loop class="wide300">Sorry, your browser doens't support embeded videos.</video> 

The code above plays fine with images and sounds.

However, the code below which I found on the net for playng mp4 plays only sounds and no images.

<video controls width="250"> 
<source src="http://myDomain.com/myMovie02.mp4" type="video/mp4">
Sorry, your browser doens't support embeded videos
</video> 

I can make mov movies to play but cannot make mp4 movies to play.
How can I make mp4 movies to play well?

Bearing in mind I know nothing about video I just copied an mp4 video link from the MDN site and it worked straight away on my mac.

<video controls width="250"> 
<source src="https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4" type="video/mp4">
Sorry, your browser doesn't support embeded videos
</video>