background-image: conic-gradient(from 45deg, red, yellow, green);

img {
  -webkit-box-reflect: right;
}

<style>
.button {
  background: linear-gradient(to top right,red,blue);
  border: none;
  color: white;
  padding: 8px 12px;
  text-align: center;
  text-decoration: none;
  display: inline-block;
  font-size: 18px;
  margin: 4px 2px;
  cursor: pointer;
  border-radius:9px;
}
</style>

<h2>CSS Buttons</h2>

<button>Default Button</button>
<a href="#" class="button">Link Button</a>
<button class="button">Button</button>
<input type="button" class="button" value="Input Button">