Border с помощью ::after и ::before

* { 
  box-sizing: border-box; 
} 
 
html, 
body { 
  padding: 0; 
  margin: 0; 
} 
 
body { 
  background: #333; 
} 
 
input { 
  height: 40px; 
  display: block; 
  width: 100%; 
  margin: 10px 0; 
  border-radius: 20px; 
  border: none; 
} 
 
legend { 
  color: #fff; 
  text-align: center; 
  padding-left: 10px; 
  padding-right: 10px; 
} 
 
form { 
  margin: 50px; 
  padding: 20px; 
} 
 
fieldset { 
  border: 2px solid #fff; 
  border-radius: 5px; 
}

<form> 
  <fieldset> 
    <legend>Personalia:</legend> 
    <input type="text"><br> 
    <input type="text"><br> 
    <input type="text"> 
  </fieldset> 
</form>

body { 
  background-color: gray; 
} 
form { 
  border: 2px solid white; 
  border-top: 0; 
  border-radius: 15px; 
  padding: 30px 10px 10px; 
  margin: 50px; 
  width: 290px; 
  position: relative; 
} 
form:after, 
form:before { 
  content: ''; 
  display: block; 
  width: 70px; 
  height: 30px; 
  border-top: 2px solid white; 
  position: absolute; 
  top: 0; 
} 
form:before { 
  border-left: 2px solid white; 
  border-top-left-radius: 15px; 
  left: -2px; 
} 
form:after { 
  border-right: 2px solid white; 
  border-top-right-radius: 15px; 
  right: -2px; 
} 
span { 
  color: white; 
  position: absolute; 
  top: -10px; 
  left: 90px; 
  font-family: Arial, sans-serif; 
  font-size: 15px; 
} 
input { 
  width: 240px; 
  height: 20px; 
  margin: 10px; 
  border-radius: 10px; 
  padding: 3px 15px; 
  border: none; 
  outline: none; 
}

<form> 
  <span>Заголовок формы:</span> 
    <input type="text"><br> 
    <input type="text"><br> 
    <input type="text"> 
</form>