İstediğini doğru anladıysam CSS before pseudo-element ile responsive yapıya uygun yapabilirsin.

<div class="ellipse">
  <span>We are good at what we do.</span>
</div>

  .ellipse {
    width: 70%;
    height: 200px;
    overflow:hidden;
    position:relative;
    display:flex;
    justify-content: center;
    align-items: center;
  }	
  .ellipse:before {
    content: "";
    display: block;
    width: 140%;
    height: 100%;
    background: #3490dc;
    border-radius: 100% / 50%;
    position: absolute;
    left: -20%;
    z-index: -1;
  }	
  .ellipse span {color:#fff;}