Cant' find anything online about this, and yet would seem obvious requirement.
The standard video block just uses a normal HTML <video> element so to make it autoplay, you can simply copy the block's view.php file to create a template and in your template, around line 22, where it says
<video controls="controls" <?php echo $posterURL ? 'poster="' . $posterURL . '"' : '' ?>
Modify it to say
<video controls="controls" autoplay <?php echo $posterURL ? 'poster="' . $posterURL . '"' : '' ?>
But it is not necessarily a good idea because browsers are more and more blocking autoplay, especially on mobile.
You should read this to have a better idea of what browsers do and a possible strategy to deal with it:https://stackoverflow.com/questions/49822790/html5-video-autoplay-no...
I usehttp://handbrake.fr/ to re-encode videos smaller for the web, and you can also do a video only export.