While I myself would be waiting here for an answer, I wonder if it can be done via CSS:

@media only screen and (orientation:portrait){
#wrapper {width:1024px}
}


@media only screen and (orientation:landscape){
#wrapper {width:1024px}
}

@Golmaal really answered this, I'm just being a bit more verbose.

<style type="text/css">
#warning-message { display: none; }
@media only screen and (orientation:portrait){
#wrapper { display:none; }
#warning-message { display:block; }
}
@media only screen and (orientation:landscape){
#warning-message { display:none; }
}
</style>


....


<div id="wrapper">
<!-- your html for your website -->
</div>
<div id="warning-message">
this website is only viewable in landscape mode
</div>

You have no control over the user moving the orientation however you can at least message them. This example will hide the wrapper if in portrait mode and show the warning message and then hide the warning message in landscape mode and show the portrait.

I don't think this answer is any better than @Golmaal , only a compliment to it. If you like this answer, make sure to give @Golmaal the credit.

Update

I've been working with Cordova a lot recently and it turns out you CAN control it when you have access to the native features.

Another Update

So after releasing Cordova it is really terrible in the end. It is better to use something like React Native if you want JavaScript. It is really amazing and I know it isn't pure web but the pure web experience on mobile kind of failed.

Try this It may be more appropriate for you

#container { display:block; }
@media only screen and (orientation:portrait){
#container {
height: 100vw;
-webkit-transform: rotate(90deg);
-moz-transform: rotate(90deg);
-o-transform: rotate(90deg);
-ms-transform: rotate(90deg);
transform: rotate(90deg);
}
}
@media only screen and (orientation:landscape){
#container {
-webkit-transform: rotate(0deg);
-moz-transform: rotate(0deg);
-o-transform: rotate(0deg);
-ms-transform: rotate(0deg);
transform: rotate(0deg);
}
}

<div id="container">
<!-- your html for your website -->
<H1>This text is always in Landscape Mode</H1>
</div>

This will automatically manage even rotation.

I had to play with the widths of my main containers:

html {
@media only screen and (orientation: portrait) and (max-width: 555px) {
transform: rotate(90deg);
width: calc(155%);
.content {
width: calc(155%);
}
}
}