HTML5 Video Seamless Looping

Heureka!

We've found the actual, real, work-around-free solution to this problem over at where I work. It explains the inconsistent behavior through multiple developers as well.

The tl;dr version is: Bitrates. Who would've guessed? What I suppose is that many people use standard values for this that usually are around 10 Mbit/s for HD videos if you use the Adobe Media Encoder. This is not sufficient. The correct value would be 18 Mbit/s or maybe even higher. 16 is still a bit janky. I cannot express how well this works. I've, by now, tried the messiest workarounds for about five hours until I found this together with our video editor.

I hope this helps everyone and saves you tons of time!

Try this:
1) edit your video this way:
[1s][2s][3s][4s][5s]
cut 1st second block of the video and append it 2x to the end like this:
[2s][3s][4s][5s][1s][1s]

2) Use code:

<video id="vid"  width="100" height="50" loop autoplay preload="true">
    <source src="something.mp4" type="video/mp4">
</video>

<!-- Goes to end of body of course -->
<script>
    var vid = document.getElementById("vid");
    vid.addEventListener("timeupdate", function () {
        if(this.currentTime >= 5.0) {
            this.currentTime = 0.0;
        }
    });
</script>

The idea behind this is to make the video seamless (the end of the video is the beginning of the video). Also, you have to make sure the video never ends. The loop attribute works with smaller video files but you see a black image at the end of the video if too large (before the next looping iteration). Essentially before the video ends, you are seeking back to 0.0s.

I hope that helps.