A shape with rounded corners and a triangle

Question

I need to make a shape like the picture below using CSS. There will be a number in the corner and content inside. Please tell me how to do it?

I was only able to do this:

.container {
  position: relative;
  width: 200px;
  height: 200px;
  background-color: #f2f2f2;
  border-top-right-radius: 80px;
  border-bottom-left-radius: 80px;
}

.background {
  position: relative;
  top: -20px;
  left: -20px;
  width: 0;
  height: 0;
  border-top: 100px solid red;
  border-right: 100px solid transparent;
}

.number {
  display: flex;
  justify-content: center;
  align-items: center;
  width: 100%;
  height: 100%;
  color: #ffffff;
  font-size: 24px;
}

<div class="container">
  <div class="background">
    <span class="number">1</span>
  </div>
</div>

Answer 1

Try this out. Basically it's just a square with one rounded corner. Then the two angled pieces are the ::before and ::after elements rotated and manually placed. It's not perfect but it's close.

body{padding:2rem; background-color: #333;}

.container {
  position: relative;
  width: 200px;
  height: 200px;
  background-color: #FCFCFC;
  border-top-right-radius: 80px;
  border-bottom-left-radius: 80px;
}

.shape {
    position: relative;
    left: -20px;
    top: -20px;
    background: #b43b41;
    width: 100px;
    height: 100px;
    border-bottom-right-radius: 100%;
}

.shape::before,
.shape::after {
  display: block;
  content: ' ';
  background: #b43b41;
  width: 30px;
  height: 30px;
  transform: rotate(45deg);
  position: absolute;
  z-index: -1;
}

.shape::before {
  top: 6.5px;
    right: -14px;
}

.shape::after {
  bottom: -14px;
  left: 6.5px;
}

.number {
  position: absolute;
  top: -10px;
  left: -10px;
  color: #fff;
  font-size: 24px;
  width: 35px;
  text-align: center;
}

<div class="container">
  <div class="shape"></div>
  <div class="number">1</div>
</div>

Answer 2

Here is a fairly close version. There is definitely some "magic numbers" going on in here, but visually it's pretty close.

body {
  font-family: sans-serif;
  background: #25272b;
  padding: 40px;
}

.shape {
  position: relative;
  width: 380px;
  height: 300px;
  border-radius: 0 140px;
  background: #fff;
}

.shape__accent {
  position: absolute;
  top: -15px;
  left: -15px;
  width: 140px;
  height: 140px;
  border-radius: 0 0 130px 0;
  background: #b43b41;
}
.shape__accent:before, .shape__accent:after {
  position: absolute;
  width: 0;
  height: 0;
  border-top: 11px solid transparent;
  border-bottom: 11px solid transparent;
  transform: rotate(-45deg);
  content: "";
}
.shape__accent:before {
  top: 0;
  right: -9px;
  border-right: 11px solid #b43b41;
}
.shape__accent:after {
  bottom: -15px;
  left: 6px;
  border-left: 11px solid #b43b41;
}

.shape__content {
  position: relative;
  font-size: 30px;
  color: #fff;
}

<div class="shape">
  <div class="shape__accent"></div>
  <span class="shape__content">13</span>
</div>

Codepen version: https://codepen.io/seanstopnik/pen/ZEmBJrL/e14888d3196306b79a82cfecbcb44eb4

Answer 3

I wanted to play around and see if this could be done with a single element, came up with this:

div{
  height:30vw; /*VW units just to show responsiveness*/
  width:30vw;  /*Change these as necessary*/
padding:20px;
  margin:50px;      
  background:#f2f2f2;
  display:inline-block;
  position:relative;
  border-radius:0 40% 0 40%;
  min-width:150px;
  min-height:150px;  
  max-width:320px;
  max-height:320px;
}
div:before, div:after{
  content:"";
  position:absolute;
  top:-5%;
  left:-5%;
  height:40%;
  width:40%;
  background:red;
  padding:inherit;
}
div:before{
  content:'1';
  color:#f2f2f2;
  border-radius:0 0 100%  0;  
}
div:after{
transform-origin:85.5%;
  transform:rotate(-45deg);
  width:calc(55% + 20px);
  z-index:-1;
}

<div></div>

So, the top red part is split into 2 parts:

• The quarter circle
• The 'shadow' of the quarter circle

Hopefully this could be used for different sizes of height and width (as long as they are kept the same square dimensions).