you can easily use border-radius property and make elements curved. (like the picture below). Just use the appropriate degree to make a circle or a curved rectangle.

HTML:

<div class="box"><p>My Box</p></div>

<div class="circle">
  <p>Hello</p>
</div>

CSS:

body {
  margin: 20px auto;
  font-family: "Lato";
  font-weight: 300;
  padding: 20px;
}

.box {
  border-radius: 4px;
  background: red;
  width: 200px;
  height: 200px;
  line-height: 50px;
  text-align: center;
  margin: 20px;
  color: white;
  font-size: 1.4rem;
  float: left;
}

.circle {
  background: red;
  width: 200px;
  height: 200px;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  color: white;
  font-size: 2rem;
  font-weight: 600;
  margin: 20px;
}

* you can use border-radius with all html elements (for example images)

* use border-radius=50%  to make a circle