To achieve the desired slant effect, utilizing clip-path is recommended, as mentioned in the comments. It is crucial to ensure that the path matches on both the inner and outer elements for the border to align properly.
.outside {
position: relative;
width: 70vmin;
height: 70vmin;
background: red;
-webkit-clip-path: polygon(0 0, 100% 10%, 100% 100%, 0% 100%);
clip-path: polygon(0 0, 100% 10%, 100% 100%, 0% 100%);
}
.inside {
position: absolute;
top: 5px;
left: 5px;
right: 5px;
bottom: 5px;
background: black;
-webkit-clip-path: polygon(0 0, 100% 10%, 100% 100%, 0% 100%);
clip-path: polygon(0 0, 100% 10%, 100% 100%, 0% 100%);
}
<div class="outside">
<div class="inside"></div>
</div>