<fieldset>
  <legend>Select pizza size:</legend>
  <label>
    <input type="radio" name="size-0" class="radio-circle" checked />
    <span>Small</span>
  </label>
  <label>
    <input type="radio" name="size-0" class="radio-circle" />
    <span>Medium</span>
  </label>
  <label>
    <input type="radio" name="size-0" class="radio-circle" />
    <span>Large</span>
  </label>
</fieldset>

<fieldset>
  <legend>Favorite X-Men:</legend>
  <label>
    <input type="radio" name="hero-1" class="radio-x" checked />
    <span>Wolverine</span>
  </label>
  <label>
    <input type="radio" name="hero-1" class="radio-x" />
    <span>Wolverine</span>
  </label>
  <label>
    <input type="radio" name="hero-1" class="radio-x" />
    <span>Wolverine</span>
  </label>
</fieldset>

fieldset {
  width: 200px;
  margin: 10px;
  display: inline-block;
}

label {
  display: block;
  float: left;
  clear: both;
  padding-right: 0.25rem;
}

label:not(:last-child) {
  margin-bottom: 0.5rem;
}

label:focus-within {
  outline: 1px dashed blue;
  outline-offset: 2px;
}

label:focus-within * {
  outline: none;
}
 
input[type="radio"] {
  -moz-appearance: none;
  -webkit-appearance: none;
  appearance: none;
  width: 1rem;
  height: 1rem;
  border-radius: 50%;
  vertical-align: bottom;
}

.radio-circle {
  border: 2px solid #147;
  background: #147;
  box-shadow: inset 0 0 0 1rem white;
  transition: box-shadow 0.25s;
}

.radio-circle:checked {
  box-shadow: inset 0 0 0 0.125rem white;
}

.radio-x {
  border: 2px solid #04a;
  background: #fff;
  transition:  background 0.5s;
  position: relative;
  overflow: hidden;
}

.radio-x:checked {
  background: #ff0;
}

.radio-x::before {
  content: "X";
  font-family: Verdana, Arial, sans-serif;
  color: #04a;
  display: block;
  height: 100%;
  width: 100%;
  text-align: center;
  line-height: 1;

  transform:  scale(0);
  transition: transform 0.5s;
}

.radio-x:checked::before {
  transform:  scale(2.3);
} 

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