Force “landscape” orientation mode

It is now possible with the HTML5 webapp manifest. See below.

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Original answer:

You can't lock a website or a web application in a specific orientation. It goes against the natural behaviour of the device.

You can detect the device orientation with CSS3 media queries like this:

@media screen and (orientation:portrait) {
    // CSS applied when the device is in portrait mode
}

@media screen and (orientation:landscape) {
    // CSS applied when the device is in landscape mode
}

Or by binding a JavaScript orientation change event like this:

document.addEventListener("orientationchange", function(event){
    switch(window.orientation) 
    {  
        case -90: case 90:
            /* Device is in landscape mode */
            break; 
        default:
            /* Device is in portrait mode */
    }
});

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Update on November 12, 2014: It is now possible with the HTML5 webapp manifest.

As explained on html5rocks.com, you can now force the orientation mode using a manifest.json file.

You need to include those line into the json file:

{
    "display":      "standalone", /* Could be "fullscreen", "standalone", "minimal-ui", or "browser" */
    "orientation":  "landscape", /* Could be "landscape" or "portrait" */
    ...
}

And you need to include the manifest into your html file like this:

<link rel="manifest" href="manifest.json">

Not exactly sure what the support is on the webapp manifest for locking orientation mode, but Chrome is definitely there. Will update when I have the info.

screen.orientation.lock('landscape');

Will force it to change to and stay in landscape mode. Tested on Nexus 5.

http://www.w3.org/TR/screen-orientation/#examples

I use some css like this (based on css tricks):

@media screen and (min-width: 320px) and (max-width: 767px) and (orientation: portrait) {
  html {
    transform: rotate(-90deg);
    transform-origin: left top;
    width: 100vh;
    height: 100vw;
    overflow-x: hidden;
    position: absolute;
    top: 100%;
    left: 0;
  }
}