Check out example codes for "thumbnail video in html". It will help you in understanding the concepts better.

Code Example 1

<!DOCTYPE html>
<html lang=en>

<head>
<title>Alison falling out of bed on Medium</title>
</head>

<body>

<h1>Alison falling out of bed on Medium</h1>

<video poster="PreviewImage.jpeg" width="640" height="480" controls="controls">
  <source src="AlisonFallingOutOfBed.webm" type="video/webm" />
  <source src="AlisonFallingOutOfBed.mp4" type="video/mp4" />
  <source src="AlisonFallingOutOfBed.theora.ogv" type="video/ogg" />
  Bummer, your  browser does not support the video tag.
</video>

</body>
</html>

Code Example 2

<video controls poster="/images/w3html5.gif">
  <source src="movie.mp4" type="video/mp4">
  <source src="movie.ogg" type="video/ogg">
  Your browser does not support the video tag.
</video>

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