You can add a button in HTML via "filter: drop-shadow". Example shown below:

body {
  background: #76d7c4;
}

button {
  text-transform: uppercase;
  padding: 10px 20px;
  color: white;
  cursor: pointer;
  background: transparent; /* no background! */
  border: 1px solid white;
  border-radius: 100px;
  
  position: relative; /* new */
}

button:after {
  content: "";
  position: absolute;
  /* Making pseudoelement the same size as container */
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  /* Inheriting border properties */
  border-radius: inherit;
  border: inherit;
  /* Applying filter with shadow */
  filter: drop-shadow(5px 5px 1px rgba(0, 0, 0, 0.25));
}
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