<!DOCTYPE html>
<style>
    * {
        border: 1px solid black;
    }

    .grid-container {
        display: grid;
        grid-template-columns: [main-start] 1fr [content-start] 1fr [content-end] 1fr [main-end];
        grid-template-rows: [main-start] 25px [content-start] 25px [content-end] 25px [main-end];
    }

    .grid-item {
        background-color: palevioletred;
    }

    .a {
        grid-column-start: main-start;
        grid-row-start: main-start;
        grid-row-end: main-end;
    }

    