When defining margins for pages, not giving any unit when setting margin:1cm 0 2cm 2cm; is not working as expected. The zero margin is ignored (margin:1cm 2cm 2cm; is applied). To get it working, we have to tell margin:1cm 0cm 2cm 2cm;.

When extending a previous @page, such as:

@page { 
    margin: 2cm;
}
@page cover { 
    margin: 0;
}
section.cover {
    page: cover;
}

The .cover section has a 2cm margin.