Borda botão gradiente com CSS (button border gradient)

/* vars */
.btn-gradient {
  --btn-gradient-border-width: 2px;
  --btn-gradient-border-radius: 6px;
  --btn-gradient-color-1: #0001ff;
  --btn-gradient-color-2: #ff0066;
}

/* code */
.btn-gradient {
  font-weight: bold;
  letter-spacing: 1px;
  color: #fff;
  position: relative;
  background: linear-gradient(to right, var(--btn-gradient-color-1), var(--btn-gradient-color-2)),
    linear-gradient(to right, var(--btn-gradient-color-1), var(--btn-gradient-color-2));
  background-position: var(--btn-gradient-border-radius) 0, var(--btn-gradient-border-radius) 100%;
  background-repeat: no-repeat;
  background-size: calc(100% - var(--btn-gradient-border-radius) - var(--btn-gradient-border-radius))
    var(--btn-gradient-border-width);
  padding: 14px 24px;
  border-radius: var(--btn-gradient-border-radius);
  border: none;
}

.btn-gradient:before,
.btn-gradient:after {
  content: "";
  display: block;
  position: absolute;
  width: var(--btn-gradient-border-radius);
  top: 0;
  bottom: 0;
}

.btn-gradient:before {
  left: 0;
  border: var(--btn-gradient-border-width) solid var(--btn-gradient-color-1);
  border-top-left-radius: var(--btn-gradient-border-radius);
  border-bottom-left-radius: var(--btn-gradient-border-radius);
  border-right-color: transparent;
}

.btn-gradient:after {
  right: 0;
  border: var(--btn-gradient-border-width) solid var(--btn-gradient-color-2);
  border-top-right-radius: var(--btn-gradient-border-radius);
  border-bottom-right-radius: var(--btn-gradient-border-radius);
  border-left-color: transparent;
}