progressive-enhancement-3.css

.layout-main {
  display: inline-block;
  display: flex;
  display: grid;
  width: 100%;
}

.layout-main__left {
  float: left;
  width: 75%;
}

.layout-main__right {
  float: left;
  width: 25%;
}

@supports (display: grid) {
  .layout-main {
    grid-template-columns: 75% 25%;
  }

  .layout-main__left,
  .layout-main__right {
    width: auto;
  }
}