Question

I got a CSS3 tag called body:before,
and I want to use JavaScript to set the height and width of body:before.

How can I do that?

Answer 1

You can’t. One possiblity is to use a class or ID:

.someClass:before {
    /*Styles here*/
}

You can then apply the someClass class to the elements in question by setting the className property:

yourElement.className = "someClass";