<!DOCTYPE html>
<meta charset="utf-8">
<style>
.container {
  display: flex;
  width: 500px;
  height: 150px;
  margin: 10px;
}

.item {
  background: yellow;
  color: red;
  flex-grow: 1 !important;
  max-width: 350px;
  max-width: 400px !important;
  padding: 10px;
}
</style>
<div class="container">
  <div class="item" style="flex-grow: 5 !important">Item</div>
</div>
