如何显示背景图像的剪辑内容并且只显示边框？

Question

This codepen https://codepen.io/Chester/pen/QPoyjN here shows how to do border animation. It works well, however they are using white background and I want to make background of the box dynamic.

<div class="rainbow">
    Rainbow border
</div>

Suppose the div already has some class which sets it's background. How do I make the ::after background use already existing background?

Is there any solution to clip background from content and only show ::before background to borders?

Here's the magic scss code:

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
}

*, *::before, *::after {
    box-sizing: border-box;
}

@keyframes rotate {
    100% {
        transform: rotate(1turn);
    }
}

.rainbow {
    position: relative;
    z-index: 0;
    width: 400px;
    height: 300px;
    border-radius: 10px;
    overflow: hidden;
    padding: 2rem;
    
    &::before {
        content: '';
        position: absolute;
        z-index: -2;
        left: -50%;
        top: -50%;
        width: 200%;
        height: 200%;
        background-color: #399953;
        background-repeat: no-repeat;
        background-size: 50% 50%, 50% 50%;
        background-position: 0 0, 100% 0, 100% 100%, 0 100%;
        background-image: linear-gradient(#399953, #399953), linear-gradient(#fbb300, #fbb300), linear-gradient(#d53e33, #d53e33), linear-gradient(#377af5, #377af5);
        animation: rotate 4s linear infinite;
    }
    
    &::after {
        content: '';
        position: absolute;
        z-index: -1;
        left: 6px;
        top: 6px;
        width: calc(100% - 12px);
        height: calc(100% - 12px);
        background: white;
        border-radius: 5px;
    }
}

Answer 1

You can use background: inherit;

 body { display: flex; justify-content: center; align-items: center; height: 100vh; } *, *::before, *::after { box-sizing: border-box; } @-webkit-keyframes rotate { 100% { transform: rotate(1turn); } } @keyframes rotate { 100% { transform: rotate(1turn); } }.rainbow { position: relative; z-index: 0; width: 400px; height: 300px; border-radius: 10px; overflow: hidden; padding: 2rem; background: pink; }.rainbow::before { content: ""; position: absolute; z-index: -2; left: -50%; top: -50%; width: 200%; height: 200%; background-color: #399953; background-repeat: no-repeat; background-size: 50% 50%, 50% 50%; background-position: 0 0, 100% 0, 100% 100%, 0 100%; background-image: linear-gradient(#399953, #399953), linear-gradient(#fbb300, #fbb300), linear-gradient(#d53e33, #d53e33), linear-gradient(#377af5, #377af5); -webkit-animation: rotate 4s linear infinite; animation: rotate 4s linear infinite; }.rainbow::after { content: ""; position: absolute; z-index: -1; left: 6px; top: 6px; width: calc(100% - 12px); height: calc(100% - 12px); background: inherit; border-radius: 5px; }

 <div class="rainbow"> Rainbow border </div>