Let’s assume there is a style like the following:

.page-L{
    position: relative;
    height: 100%;
    width: 30%;
    transform-origin: left;
    transform: rotateY(α);
}
.page-R{
    position: relative;
    margin: auto;
    height: 100%;
    width: 70%;
}

enter image description here

These two elements are brother elements.

As you can see, you can clearly see that the element ‘page-L’ will undergo a deformation, and this will cause a visual change to its right side.

What should I do to obtain its changed value (height) through JavaScript and apply it to the element ‘page-R’?