I'm trying to mark left and right margin black for a specific element, for ex. insert element, based on which page it is. This is not working.

@page :left {
margin: 0.5in, 0.75in, 0.5in, 0.5in;
@left {
content: element(insert-marker);
border-right: 1px solid black;
width: 1px;
height: 1.5em;
margin-left: 0.25in;
}
}

@page :right {
margin: 0.5in, 0.5in, 0.5in, 0.75in;
@right {
content: element(insert-marker);
border-left: 1px solid black;
width: 1px;
height: 1.5em;
margin-right: 0.25in;
}
}

Here's an overview of the page margin boxes:

https://css4.pub/2024/running-headers/#page-margin-boxes

Edited 16 Aug 2025 by howcome