Cut Corners using CSS

Question

I\'m looking to \"cut\" the top left corner of a div, like if you had folded the corner of a page down.

I\'d like to do it in pure CSS, are there any methods?

Answer 1

According to Harry's linear-gradient solution (answered Oct 14 '15 at 9:55), it says that opacity background isn't possible, I tried it and yep, it isn't.

But! I found a workaround. No it's not super optimised, but it worked. So here's my solution. Since Harry doesn't use pseudo element, we can achieve this by creating one.

Set position relative to the container and create a pseudo element with the same linear-gradient properties. In other words, just clone it. Then put a transparent background for the container, and lets say a black background for the clone. Put a position absolute on it, a z-index of -1 and an opacity value (ie. 50%). It will do the job. Again it's a workaround and it's not perfect but it works just fine.

.cut-corner {
    position: relative;
    color: white;
    background-repeat: no-repeat;
    background-image: linear-gradient(white, white), linear-gradient(white, white), linear-gradient(white, white), linear-gradient(white, white), linear-gradient(to bottom left, transparent calc(50% - 1px), white calc(50% - 1px), white calc(50% + 1px), transparent calc(50% + 1px)), linear-gradient(transparent, transparent), linear-gradient(transparent, transparent);
    background-size: 2px 100%, 2px 100%, 100% 2px, 100% 2px, 25px 25px, 100% 100%, 100% 100%;
    background-position: 0% 0%, 100% 25px, -25px 0%, 0px 100%, 100% 0%, -25px 0%, 100% 25px;
}
.cut-corner:after {
    content: "";
    position: absolute;
    left: 0;
    bottom: 0;
    right: 0;
    top: 0;
    z-index: -1;
    opacity: 0.5;
    background-repeat: no-repeat;
    background-image: linear-gradient(white, white), linear-gradient(white, white), linear-gradient(white, white), linear-gradient(white, white), linear-gradient(to bottom left, transparent calc(50% - 1px), white calc(50% - 1px), white calc(50% + 1px), black calc(50% + 1px)), linear-gradient(black, black), linear-gradient(black, black);
    background-size: 2px 100%, 2px 100%, 100% 2px, 100% 2px, 25px 25px, 100% 100%, 100% 100%;
    background-position: 0% 0%, 100% 25px, -25px 0%, 0px 100%, 100% 0%, -25px 0%, 100% 25px;
}

/* Just for demo */

div {
  padding: 10px;
}
body{
 background-image: radial-gradient(circle, #3F9CBA 0%, #153346 100%);
}

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