2020 answer

HTML 5 provides no way to make a video fullscreen, but the parallel Fullscreen API defines an API for elements to display themselves fullscreen.

This can be applied to any element, including videos.

Browser support is good, but Internet Explorer and Safari need prefixed versions.

An external demo is provided as Stack Snippet sandboxing rules break it.

<div id="one">
    One
</div>

<div id="two">
    Two
</div>

<button>one</button>
<button>two</button>

div {
    width: 200px;
    height: 200px;
}
#one { background: yellow; }
#two { background: pink; }

addEventListener("click", event => {
    const btn = event.target;
    if (btn.tagName.toLowerCase() !== "button") return;
    const id = btn.textContent;
    const div = document.getElementById(id);
    if (div.requestFullscreen) 
        div.requestFullscreen();
    else if (div.webkitRequestFullscreen) 
        div.webkitRequestFullscreen();
    else if (div.msRequestFullScreen) 
      div.msRequestFullScreen();
});

2012 answer

HTML 5 provides no way to make a video fullscreen, but the parallel Fullscreen specification supplies the requestFullScreen method which allows arbitrary elements (including <video> elements) to be made fullscreen.

It has experimental support in a number of browsers.

2009 answer

Note: this has since been removed from the specification.

From the HTML5 spec (at the time of writing: June '09):

User agents should not provide a public API to cause videos to be shown full-screen. A script, combined with a carefully crafted video file, could trick the user into thinking a system-modal dialog had been shown, and prompt the user for a password. There is also the danger of "mere" annoyance, with pages launching full-screen videos when links are clicked or pages navigated. Instead, user-agent specific interface features may be provided to easily allow the user to obtain a full-screen playback mode.

Browsers may provide a user interface, but shouldn't provide a programmable one.

I think that if we want to have a open way to view videos in our browsers without any closed source plugins (and all the security breaches that comes with the history of the flash plugin...). The tag has to find a way to activate full screen.. We could handle it like flash does: to do fullscreen, it has to be activated by a left click with your mouse and nothing else, I mean it's not possible by ActionScript to launch fullscreen at the loading of a flash by example.

I hope I've been clear enough: After all, I'm only a french IT student, not an english poet :)

See Ya!

You can change the width and height to be 100%, but it won't cover the browser chrome or the OS shell.

Design decision is because HTML lives inside the browser window. Flash plugins aren't inside the window, so they can go full screen.

This makes sense, otherwise you could make img tags that covered the shell, or make h1 tags so the whole screen was a letter.

No, it is not possible to have fullscreen video in html 5. If you want to know reasons, you're lucky because the argument battle for fullscreen is fought right now. See WHATWG mailing list and look for the word "video". I personally hope that they provide fullscreen API in HTML 5.

An alternative solution would be to have to browser simply provide this option on the contextual menu. No need to have Javascript to do this, though I could see when it would be useful.

In the mean time an alternative solution would simply be to maximise the window (Javascript can provide screen dimensions) and then maximise the video within it. Give it a go and then simply see if the results are acceptable to your users.

If you have option to define your site as progressive web app (PWA), then there is also option to use display: "fullscreen" under manifest.json. But this will only work if user adds/installs your webapp to home screen and opens it up from there.