Simple grid

HTML

<!-- Row -->
<div class="simple-grid">
    <!--Cell with given width. Replace 25% with whatever you want -->
    <div class="simple-grid__cell simple-grid__cell--1/4">25%</div>

    <!-- Cell that takes remaining width -->
    <div class="simple-grid__cell simple-grid__cell--fill">...</div>
</div>

CSS

.simple-grid {
    display: flex;

    margin-left: -0.25rem;
    margin-right: -0.25rem;
}

.simple-grid__cell {
    padding-left: 0.25rem;
    padding-right: 0.25rem;
}

.simple-grid__cell--fill {
    flex: 1;
}

/* Cell with given width */
.simple-grid__cell--1\/2 {
    flex: 0 0 50%;
}
.simple-grid__cell--1\/3 {
    flex: 0 0 33.3333333%;
}
.simple-grid__cell--1\/4 {
    flex: 0 0 25%;
}