Why isn't the section overlay showing on Macbook?

Question

HTML

On the main screen, in the section main.
Not shown on all other devices/browsers.
Here is the site: styazhkapola72.ru
HTML:

<div class="main-overlay">
</div>

CSS:

.main-overlay {
    background-image: url(../img/backgrounds/overlay.png);
    background-position: top right;
    -webkit-background-size: cover;
    background-size: cover;
    height: 100%;
    z-index: -1;
}

Answer 1

set main z-index: 2;