Why doesn't play event get triggered when the video with autoplay first starts playing after loading the page?

```js
document.getElementById("vid").addEventListener('play', function, false);
```

```html
<video id="vid" src="video.mp4" type="video/mp4" autoplay></video>
```

It should fire the play event I think, but doesn't. 

Is this by design? and if so, why?

Manually pressing play works normally.