How can I show only corner borders?

Question

Here is an idea using gradient and CSS variables where you can easily control the shape of your border:

.box {
  --b:5px;   /* thickness of the border */
  --c:red;   /* color of the border */
  --w:20px;  /* width of border */
  

  border:var(--b) solid transparent; /* space for the border */
  --g:#0000 90deg,var(--c) 0;
  background:
    conic-gradient(from 90deg  at top    var(--b) left  var(--b),var(--g)) 0    0,
    conic-gradient(from 180deg at top    var(--b) right var(--b),var(--g)) 100% 0,
    conic-gradient(from 0deg   at bottom var(--b) left  var(--b),var(--g)) 0    100%,
    conic-gradient(from -90deg at bottom var(--b) right var(--b),var(--g)) 100% 100%;
  background-size:var(--w) var(--w);
  background-origin:border-box;
  background-repeat:no-repeat;
  
  /*Irrelevant code*/  
  width:200px;
  height:100px;
  box-sizing:border-box;
  margin:5px;
  display:inline-flex;
  font-size:30px;
  justify-content:center;
  align-items:center;
  line-height:90px;
}

<div class="box">
some content
</div>

<div class="box" style="--c:blue;--w:40px;--b:2px">
some content
</div>

<div class="box" style="--c:green;--w:30%;--b:8px">
some content
</div>

<div class="box" style="--c:black;--w:50%;--b:3px">
some content
</div>

<div class="box" style="--c:purple;--w:10px;--b:10px">
some content
</div>

<div class="box" style="--c:orange;--w:calc(50% - 10px);--b:4px">
some content
</div>

CSS corner border only

You can also have a complex coloration if you combine this with mask:

.box {
  --b:5px;   /* thickness of the border */
  --c:red;   /* color of the border */
  --w:20px;  /* width of border */
  

  padding:var(--b); /* space for the border */
  
  position:relative;
  /*Irrelevant code*/  
  width:200px;
  height:100px;
  box-sizing:border-box;
  margin:5px;
  display:inline-flex;
  font-size:30px;
  justify-content:center;
  align-items:center;
  line-height:90px;
}
.box::before {
  content:"";
  position:absolute;
  top:0;
  left:0;
  right:0;
  bottom:0;
  background:var(--c,red);
  --g:#0000 90deg,#000 0;
  -webkit-mask:
    conic-gradient(from 90deg  at top    var(--b) left  var(--b),var(--g)) 0    0,
    conic-gradient(from 180deg at top    var(--b) right var(--b),var(--g)) 100% 0,
    conic-gradient(from 0deg   at bottom var(--b) left  var(--b),var(--g)) 0    100%,
    conic-gradient(from -90deg at bottom var(--b) right var(--b),var(--g)) 100% 100%;
  -webkit-mask-size:var(--w) var(--w);
  background-origin:border-box;
  -webkit-mask-repeat:no-repeat;
  mask:
    conic-gradient(from 90deg  at top    var(--b) left  var(--b),var(--g)) 0    0,
    conic-gradient(from 180deg at top    var(--b) right var(--b),var(--g)) 100% 0,
    conic-gradient(from 0deg   at bottom var(--b) left  var(--b),var(--g)) 0    100%,
    conic-gradient(from -90deg at bottom var(--b) right var(--b),var(--g)) 100% 100%;
  mask-size:var(--w) var(--w);
  mask-repeat:no-repeat;
}

<div class="box">
some content
</div>

<div class="box" style="--c:repeating-linear-gradient(45deg,red,blue);--w:40px;--b:2px">
some content
</div>

<div class="box" style="--c:repeating-linear-gradient(90deg,#000 0 5px,transparent 5px 10px);--w:30%;--b:8px">
some content
</div>

<div class="box" style="--c:conic-gradient(red,green,yellow);--w:50%;--b:3px">
some content
</div>

<div class="box" style="--c:purple;--w:10px;--b:10px">
some content
</div>

<div class="box" style="--c:repeating-linear-gradient(45deg,orange 0 5px,blue 5px 10px);--w:calc(50% - 10px);--b:4px">
some content
</div>

CSS corner only multi-color

And why not with radius:

.box {
  --b:5px;   /* thickness of the border */
  --c:red;   /* color of the border */
  --w:20px;  /* width of border */
  --r:25px;  /* radius */
  

  padding:var(--b); /* space for the border */
  
  position:relative;
  /*Irrelevant code*/  
  width:200px;
  height:100px;
  box-sizing:border-box;
  margin:5px;
  display:inline-flex;
  font-size:30px;
  justify-content:center;
  align-items:center;
  line-height:90px;
}
.box::before {
  content:"";
  position:absolute;
  top:0;
  left:0;
  right:0;
  bottom:0;
  background:var(--c,red);
  padding:var(--b);
  border-radius:var(--r);
  -webkit-mask:
    linear-gradient(#fff 0 0) top   /calc(100% - 2*var(--w)) var(--b),
    linear-gradient(#fff 0 0) bottom/calc(100% - 2*var(--w)) var(--b),
    linear-gradient(#fff 0 0) left  /var(--b) calc(100% - 2*var(--w)),
    linear-gradient(#fff 0 0) right / var(--b) calc(100% - 2*var(--w)),
    linear-gradient(#fff 0 0) content-box,
    linear-gradient(#fff 0 0);
  -webkit-mask-composite:destination-out;
  -webkit-mask-repeat:no-repeat;
  mask:
    linear-gradient(#fff 0 0) top   /calc(100% - 2*var(--w)) var(--b),
    linear-gradient(#fff 0 0) bottom/calc(100% - 2*var(--w)) var(--b),
    linear-gradient(#fff 0 0) left  /var(--b) calc(100% - 2*var(--w)),
    linear-gradient(#fff 0 0) right / var(--b) calc(100% - 2*var(--w)),
    linear-gradient(#fff 0 0) content-box,
    linear-gradient(#fff 0 0);
  mask-composite:exclude;
  mask-repeat:no-repeat;
}

<div class="box">
some content
</div>

<div class="box" style="--c:repeating-linear-gradient(45deg,red,blue);--w:40px;--b:2px;--r:40px;">
some content
</div>

<div class="box" style="--c:repeating-linear-gradient(90deg,#000 0 5px,transparent 5px 10px);--w:30%;--b:8px">
some content
</div>

<div class="box" style="--c:conic-gradient(red,green,yellow);--w:50%;--b:3px">
some content
</div>

<div class="box" style="--c:purple;--w:10px;--b:10px;--r:0px">
some content
</div>

<div class="box" style="--c:repeating-linear-gradient(45deg,orange 0 5px,blue 5px 10px);--w:calc(50% - 10px);--b:4px;--r:10px">
some content
</div>

Corner only border with radius and CSS gradient

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