CSS Tip

Fade content inside border using mask

Create a fading content while keeping the border visible using CSS mask. It works with border-radius and no need to know the value of border.

A fading content

.box {
  border: 4px solid orange; /* we don't need to know the border-width */
  mask:
    linear-gradient(#000 0 0), 
    /* fade only the padding area 
       (in the opposite direction) */
    linear-gradient(#0000,#000 90%) padding-box;
  mask-composite: exclude;
}

See the Pen Fading content inside border II by Temani Afif (@t_afif) on CodePen.

A different idea without mask-composite but it won't work with border-radius and you need to know the border-width.

.box {
  --b: 4px; /* border thickness */
  
  border: var(--b) solid orange;
  mask:
    /* keep the border visible */
    conic-gradient(from 90deg at calc(2*var(--b)) calc(2*var(--b)),#0000 25%,#000 0)
    calc(-1*var(--b)) calc(-1*var(--b)), 
    /* fade the content */
    linear-gradient(#000,#0000 90%);
}

Online demo: codepen.io/t_afif/pen/YzObqzg