Border with gradient and radius.

The border-image property (the proper way of creating a gradient border) is not compatible with the border-radius property:

.container {
  border-width: var(--border-width);
  border-style: solid;
  border-image: linear-gradient(135deg, var(--color-1), var(--color-2)) 1 stretch;
  border-radius: var(--border-radius); /* 👈 this has no effect */
}

We can use a clip-path to round the outer corners. But the inner corners remain square:

.container {
  border-width: var(--border-width);
  border-style: solid;
  border-image: linear-gradient(135deg, var(--color-1), var(--color-2)) 1 stretch;
  clip-path: inset(0px round var(--border-radius));
}

So, a nice-looking border with both a gradient and a radius requires a little more CSS (some calc -ulations and a pseudo-element):

.container {
  width: calc(100% - (2 * var(--border-width)));
  margin: var(--border-width);
  position: relative;
  border-radius: calc(var(--border-radius) - var(--border-width));
  background-color: var(--background);
}

.container::before {
  content: "";
  position: absolute;
  top: calc(var(--border-width) * -1);
  right: calc(var(--border-width) * -1);
  bottom: calc(var(--border-width) * -1);
  left: calc(var(--border-width) * -1);
  z-index: -1;
  border-radius: var(--border-radius);
  background-image: linear-gradient(135deg, var(--color-1), var(--color-2))
}

Update

A colleague showed me Adam Argyle 's awesome pen achieving the same effect by using two backgrounds and clipping them to different box-models 🤯

.container {
  border: var(--border-width) solid transparent;
  background-clip:
    padding-box,
    border-box;
  background-image:
    linear-gradient(var(--background), var(--background)),
    linear-gradient(135deg, var(--color-1), var(--color-2));
  background-origin: border-box;
}

Post-script

The above are, however, still work-arounds or hacks. Neither solution actually uses the border-image property.

We frequently use work-arounds to achieve stuff that isn't available with properties, but once the property has landed it feels a bit odd to have to circumvent it?

Shouldn't we simply be able to write:

.container {
  border-width: var(--border-width);
  border-image: linear-gradient(135deg, var(--color-1), var(--color-2)) 1 stretch;
  border-radius: var(--border-radius);
}

The spec does not explain why the border-radius does not affect a border-image . I wonder if it is to do with the border-image-outset property or difficulty implementing (the outline property was not affected by the border-radius property in the beginning).

If you know the answer, please comment on this pen.