<div class="box">
  Look ma, gradient border with rounded corners and transparent background!!!
</div>

html {
  height: 100%;
}

body {
  background-color: rgb(240 241 245);
  background-image: linear-gradient(rgb(210 213 219) 1px, transparent 1px), linear-gradient(to right, rgb(210 213 219) 1px, rgb(240 241 245) 1px);
  background-size: 20px 20px;
  display: grid;
  height: 100%;
  margin: 0;
  font-family: sans-serif;
  place-items: center;
  color: rgb(22 25 30);
}

/* the interesting part follows below: */

.box {
  --box-border--border: linear-gradient(105deg, rgb(255 46 144) 0%, rgb(61 35 185) 100%);
  
  /* classic 9-slide-scaling with 2px border 4px rounded corners. change `rx` and `ry` parameters to adjust border-radius */
  --box--border__top-left: url("data:image/svg+xml,<svg width='10' height='10' viewBox='0 0 10 10' fill='none' xmlns='http://www.w3.org/2000/svg'><rect x='1' y='1' width='18' height='18' rx='4' ry='4' stroke='%23000' stroke-width='2' /></svg>");
  --box--border__top: url("data:image/svg+xml,<svg preserveAspectRatio='none' width='100' height='10' viewBox='0 0 100 10' fill='none' xmlns='http://www.w3.org/2000/svg'><line x1='-1' y1='1' x2='101' y2='1' stroke='%23000' stroke-width='2'/></svg>");
  --box--border__top-right: url("data:image/svg+xml,<svg width='10' height='10' viewBox='0 0 10 10' fill='none' xmlns='http://www.w3.org/2000/svg'><rect x='-9' y='1' width='18' height='18' rx='4' ry='4' stroke='black' stroke-width='2' /></svg>");
  --box--border__left: url("data:image/svg+xml,<svg preserveAspectRatio='none' width='10' height='100' viewBox='0 0 10 100' fill='none' xmlns='http://www.w3.org/2000/svg'><line x1='1' y1='-1' x2='1' y2='101' stroke='%23000' stroke-width='2'/></svg>");
  --box--border__right: url("data:image/svg+xml,<svg preserveAspectRatio='none' width='10' height='100' viewBox='0 0 10 100' fill='none' xmlns='http://www.w3.org/2000/svg'><line x1='9' y1='-1' x2='9' y2='101' stroke='%23000' stroke-width='2'/></svg>");
  --box--border__bottom-left: url("data:image/svg+xml,<svg width='10' height='10' viewBox='0 0 10 10' fill='none' xmlns='http://www.w3.org/2000/svg'><rect x='1' y='-9' width='18' height='18' rx='4' ry='4' stroke='%23000' stroke-width='2' /></svg>");
  --box--border__bottom: url("data:image/svg+xml,<svg preserveAspectRatio='none' width='100' height='10' viewBox='0 0 100 10' fill='none' xmlns='http://www.w3.org/2000/svg'><line x1='-1' y1='9' x2='101' y2='9' stroke='%23000' stroke-width='2'/></svg>");
  --box--border__bottom-right: url("data:image/svg+xml,<svg width='10' height='10' viewBox='0 0 10 10' fill='none' xmlns='http://www.w3.org/2000/svg'><rect x='-9' y='-9' width='18' height='18' rx='4' ry='4' stroke='black' stroke-width='2' /></svg>");
  
  padding: 1rem;
  position: relative;
  resize: both;
  overflow: hidden;
  font-weight: 700;
  width: 100%;
  max-width: 30ch;
}

.box::before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: var(--box-border--border);
  mask:
    var(--box--border__top-left) 0 0 / .625rem .625rem,
    var(--box--border__top) .625rem 0 / calc(100% - 1.25rem) .625rem,
    var(--box--border__top-right) 100% 0 / .625rem .625rem,
    var(--box--border__left) 0 .625rem / .625rem calc(100% - 1.25rem),
    var(--box--border__right) 100% .625rem / .625rem calc(100% - 1.25rem),
    var(--box--border__bottom-left) 0 100% / .625rem .625rem,
    var(--box--border__bottom) .625rem 100% / calc(100% - 1.25rem) .625rem,
    var(--box--border__bottom-right) 100% 100% / .625rem .625rem;
  mask-repeat: no-repeat;
}