How to make 3-corner-rounded triangle in CSS

I'd like to achieve a custom-colored shape like this using no Javascript:

Currently I'm overlaying an image of the 'frame' over an orange rectangular div, but this is pretty hacky. I suppose I could use a dynamically generated canvas element, but that not only requires JS, but HTML5 canvas support. Any ideas?

My best attempt: http://dabblet.com/gist/4592062

Pixel perfection at any size, uses simpler math than Ana's original solution, and is more intuitive in my opinion :)

.triangle {
	position: relative;
	background-color: orange;
	text-align: left;
}
.triangle:before,
.triangle:after {
	content: '';
	position: absolute;
	background-color: inherit;
}
.triangle,
.triangle:before,
.triangle:after {
	width:  10em;
	height: 10em;
	border-top-right-radius: 30%;
}

.triangle {
	transform: rotate(-60deg) skewX(-30deg) scale(1,.866);
}
.triangle:before {
	transform: rotate(-135deg) skewX(-45deg) scale(1.414,.707) translate(0,-50%);
}
.triangle:after {
	transform: rotate(135deg) skewY(-45deg) scale(.707,1.414) translate(50%);
}

<div class="triangle"></div>

.triangle, .triangle:before, .triangle:after { width: 4em; height: 4em; }
.triangle {
	overflow: hidden;
	position: relative;
	margin: 7em auto 0;
	border-radius: 20%;
	transform: translateY(50%) rotate(30deg) skewY(30deg) scaleX(.866);
	cursor: pointer;
	pointer-events: none;
} 
.triangle:before, .triangle:after {
	position: absolute;
	background: orange;
	pointer-events: auto;
	content: '';
}
.triangle:before {
	border-radius: 20% 20% 20% 53%;
	transform: scaleX(1.155) skewY(-30deg) rotate(-30deg) translateY(-42.3%) 
			skewX(30deg) scaleY(.866) translateX(-24%);
}
.triangle:after {
	border-radius: 20% 20% 53% 20%;
	transform: scaleX(1.155) skewY(-30deg) rotate(-30deg) translateY(-42.3%) 
			skewX(-30deg) scaleY(.866) translateX(24%);
}

/** extra styles to show how it works **/

.triangle:hover { overflow: visible; }
.triangle:hover:before, .triangle:hover:after { background: none; }
.triangle:hover, .triangle:hover:before, .triangle:hover:after {
	border: dashed 1px;
}

<div class='triangle'></div>

The idea is really simple: you first apply a series of transforms to your .triangle element (which has overflow: hidden; - you can remove that to see what happens ;) ) in order to get a rhombus.

Then you apply the same transforms to the :before and :after pseudo-elements, plus a few more to make them rhomboidal as well.

And in the end, you have three rhombuses which intersect, the orange shape being their intersection. Hover the triangle to see the intersecting shapes ;)

It scales nicely, you just have to change the width and the height of the .triangle element.

For Firefox, Chrome and Safari, only the orange triangle with rounded corners is sensitive to hover (thanks to pointer-events: none; on the .triangle element and pointer-events: auto; on the pseudo-elements). Otherwise, this could be achieved by wrapping .triangle in an element having the same width and height (and the same border-radius) and overflow: hidden;.

---

Notes

• You could also do it with CSS gradients. However, unlike 2D transforms, CSS gradients won't work in IE9.
• I'd wish I didn't have to unskew the pseudo-elemets which inherit the skew from their parent only to skew them again after a rotation, but it doesn't seem to work otherwise.

Use an image of some sort. That's what images are for. If you need it to scale, SVG is a good choice, otherwise, just use a png as a background, or an <img> element if it's part of content.

If you absolutely must have it in a CSS file, you could try data: urls (not supported in IE7 and below).

First we create the triangle using clip-path:

.triangle {
  display: inline-block;
  width: 150px;
  color:orange;
}

.triangle::before {
  content: "";
  display: block;
  padding-top: 86%;
  background: currentColor;
  clip-path: polygon(50% 0, 100% 100%, 0 100%);
}

<div class="triangle"></div>

And then we apply and SVG filter inspired from this article

.triangle {
  display: inline-block;
  width: 150px;
  color:orange;
  filter: url('#goo');
}

.triangle::before {
  content: "";
  display: block;
  padding-top: 86%;
  background: currentColor;
  clip-path: polygon(50% 0, 100% 100%, 0 100%);
}

<div class="triangle"></div>
<div class="triangle" style="color:red;width:200px;"></div>
<div class="triangle" style="color:blue;width:250px;"></div>


<svg style="visibility: hidden; position: absolute;" width="0" height="0" xmlns="http://www.w3.org/2000/svg" version="1.1">
  <defs>
        <filter id="goo"><feGaussianBlur in="SourceGraphic" stdDeviation="8" result="blur" />    
            <feColorMatrix in="blur" mode="matrix" values="1 0 0 0 0  0 1 0 0 0  0 0 1 0 0  0 0 0 19 -9" result="goo" />
            <feComposite in="SourceGraphic" in2="goo" operator="atop"/>
        </filter>
    </defs>
</svg>

To control the radius, we simply adjust the stdDeviation of the filter

---

Considering this, you can make it working with any kind of triangle and even a random shape:

.triangle {
  display: inline-block;
  width: 150px;
  color:orange;
  filter: url('#goo');
}

.triangle::before {
  content: "";
  display: block;
  padding-top: 86%;
  background: currentColor;
  clip-path: polygon(50% 0, 100% 100%, 0 100%);
}

.triangle.type2::before {
  padding-top: 70%;
  clip-path: polygon(0 0, 100% 100%, 0 100%);
}

.triangle.type3::before {
  padding-top: 100%;
  clip-path: polygon(50% 0, 80% 100%, 0 70%);
}

.triangle.hex::before {
  padding-top: 100%;
  clip-path: polygon(25% 0%, 75% 0%, 100% 50%, 75% 100%, 25% 100%, 0% 50%);
}

<div class="triangle"></div>
<div class="triangle type2" style="color:red;"></div>
<div class="triangle type3" style="color:blue;"></div>
<div class="triangle hex" style="color:purple;"></div>



<svg style="visibility: hidden; position: absolute;" width="0" height="0" xmlns="http://www.w3.org/2000/svg" version="1.1">
  <defs>
        <filter id="goo"><feGaussianBlur in="SourceGraphic" stdDeviation="8" result="blur" />    
            <feColorMatrix in="blur" mode="matrix" values="1 0 0 0 0  0 1 0 0 0  0 0 1 0 0  0 0 0 19 -9" result="goo" />
            <feComposite in="SourceGraphic" in2="goo" operator="atop"/>
        </filter>
    </defs>
</svg>

Worth to note that we can easily add complex background to the shapes:

.triangle {
  display: inline-block;
  width: 150px;
  filter: url('#goo');
}

.triangle::before {
  content: "";
  display: block;
  padding-top: 86%;
  background: var(--b,orange);
  clip-path: polygon(50% 0, 100% 100%, 0 100%);
}

.triangle.type2::before {
  padding-top: 70%;
  clip-path: polygon(0 0, 100% 100%, 0 100%);
}

.triangle.type3::before {
  padding-top: 100%;
  clip-path: polygon(50% 0, 80% 100%, 0 70%);
}

.triangle.hex::before {
  padding-top: 100%;
  clip-path: polygon(25% 0%, 75% 0%, 100% 50%, 75% 100%, 25% 100%, 0% 50%);
}

<div class="triangle"></div>
<div class="triangle type2" style="--b:linear-gradient(red,blue);"></div>
<div class="triangle type3" style="--b:conic-gradient(green,pink,green);"></div>
<div class="triangle hex" style="--b:url(https://picsum.photos/id/1067/200/200) center/cover;"></div>



<svg style="visibility: hidden; position: absolute;" width="0" height="0" xmlns="http://www.w3.org/2000/svg" version="1.1">
  <defs>
        <filter id="goo"><feGaussianBlur in="SourceGraphic" stdDeviation="8" result="blur" />    
            <feColorMatrix in="blur" mode="matrix" values="1 0 0 0 0  0 1 0 0 0  0 0 1 0 0  0 0 0 19 -9" result="goo" />
            <feComposite in="SourceGraphic" in2="goo" operator="atop"/>
        </filter>
    </defs>
</svg>

Ana's answer inspired me to try another approach, one that's just as far from perfect, but is at least symmetrical. Here's a preview at real-size and blown up. It's simply a border-hack trangle wrapped in a clipping circle/border-radius:

And the code (adjust the overall size via single font-size property):

.triangle {
    font-size: .8em;
    position: relative;
    width: 3.8em;
    height: 3.8em;
    text-align: center;
    margin: 10% auto 0;
    overflow: hidden;
    border-radius: 100%;
} 
.triangle:before {
    content: '';
    position: absolute;
    width:0;
    height: 0;
    border: solid 2em transparent;
    border-bottom-color: orange;
    border-bottom-width: 3.2em;
    border-top-width: 0;
    margin: -.3em -2em;
}

Play with it here: http://dabblet.com/gist/4590714

Played around with Murray Smiths most upvoted version. Wrote it as a Stylus mixin and fixed some margin issues and added a direction option. The mixin also scales the triangle to somewhat pixelperfect size. Not tested very well. Use with care

http://codepen.io/perlundgren/pen/VYGdwX

    triangle(direction = up, color = #333, size = 32px)
        position: relative
        background-color: color
        width:  2*(round(size/3.25))
        height: 2*(round(size/3.25))
        border-top-right-radius: 30%
        &:before,
        &:after
          content: ''
          position: absolute
          background-color: inherit
          width:  2*(round(size/3.25))
          height: 2*(round(size/3.25))
          border-top-right-radius: 30%

        if direction is up
          transform: rotate(-60deg) skewX(-30deg) scale(1,.866)
          margin: (@width/4) (@width/2.5) (@width/1.2) (@width/2.5)

        if direction is down
          transform: rotate(-120deg) skewX(-30deg) scale(1,.866)
          margin: 0 (@width/1.5) (@width/1.5) (@width/6)

        if direction is left
          transform: rotate(-30deg) skewX(-30deg) scale(1,.866)
          margin: (@width/5) 0 (@width) (@width/1.4)

        if direction is right
          transform: rotate(-90deg) skewX(-30deg) scale(1,.866)
          margin: (@width/5) (@width/1.4) (@width) 0

        &:before
          transform: rotate(-135deg) skewX(-45deg) scale(1.414,.707) translate(0,-50%)
        &:after
          transform: rotate(135deg) skewY(-45deg) scale(.707,1.414) translate(50%)

and then just add the mixin to your class

    .triangle
      &.up
        triangle()
      &.down
        triangle(down)
      &.left
        triangle(left)
      &.right
        triangle(right)

-- Simplified version --

In my case I needed text to accompany the triangle icon with three rounded corners, however the overflow: hidden; suggested simply did not work because the text was ultimately rendered hidden.

End Result: ...Demo: https://jsfiddle.net/allenski/7p4tbznr/

I was able to achieve similar mask by using clip-path. Note: does not work in IE; however most already stopped supporting IE, especially since Microsoft did. Works great in their new Edge browser.

HTML:

<span class="warning">
    Mandatory
</span>

CSS:

.warning {
    position: relative;
    display: inline-block;
    font-weight: bold;
    color: #FF5500;
}
.warning:before {
    position: absolute;
    top: 50%;
    right: 12px;
    font-size: 18px;
    font-weight: bold;
    color: #FFFFFF;
    transform: translateY(-36%);
    text-shadow: 0 0 7px #111111;
    z-index: 1;
    content: '!';
}
.warning:after {
    display: inline-block;
    margin-left: 3px;
    font-size: 5px;
    border: solid 3em transparent;
    border-top-width: 0;
    border-bottom-width: 5em;
    border-bottom-color: #FF5500;
    clip-path: circle(54% at 50% 69%);
    vertical-align: bottom;
    content: '';
}

In the CSS, the triangle is the :after pseudo-element.

I saw there was someone asking for an isosceles triangle and through some tampering with the accepted answer above I found how to manipulate it to get what I wanted considering I needed the same. This should help anyone looking for a slight change in the rounded corner triangle.

You'll notice that I separated out the width, height, and border-top-right-radius then proceeded to change the border-top-right-radius to shape the corners. The only other thing I changed was the transform property on the element directly. You can shape it how you see fit, but those seem to be the only necessary changes.

.diff-arrow {
  margin-left:30px;
  position: relative;
  background-color: #20C0F1;
  text-align: left;
  width: 10em;
  height: 10em;
  border-top-right-radius: 20%;
}

.diff-arrow:before,
.diff-arrow:after {
  content: '';
  position: absolute;
  background-color: inherit;
  width: 10em;
  height: 10em;
  border-top-right-radius: 15%;
}

.diff-arrow {
  transform: rotate(-45deg) skewX(0deg) scale(0.5);
}

.diff-arrow:before {
  transform: rotate(-135deg) skewX(-45deg) scale(1.414, .707) translate(0, -50%);
}

.diff-arrow:after {
  transform: rotate(135deg) skewY(-45deg) scale(.707, 1.414) translate(50%);
}

<div class="diff-arrow"></div>

Started with the typical border triangle and then added svg filter to the triangle.

.c-paper-plane {
    position: relative;
}
.scene {
    transform-style: preserve-3d;
    transform: rotate3d(0.2, -1, -0.8, -177deg);
}
svg.paper-plane {
    transform: rotateX(0) translateZ(-3px);
    transform-origin: center;
}
.paper-tail {
    transform: rotateX(0deg) translate(31px, 10px) translateZ(-5px);
    transform-origin: center;
    fill: grey;
}
.paper-tail-div {
    transform: rotateX(269deg) translate(237px, 55px) translateZ(139px);
    fill: grey;
    transform-origin: center;
    position: absolute;
    top: 0;
    width: 0;
    height: 0;
    border-top: 100px solid red;
    border-right: 150px solid transparent;
    filter: url('#goo');
}

<div class="c-paper-plane">
  <div class="scene">
    <svg id="paper-plane" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 139.549 79.269">
                <defs>
                    <filter id="goo"><feGaussianBlur in="SourceGraphic" stdDeviation="8" result="blur" />    
                        <feColorMatrix in="blur" mode="matrix" values="1 0 0 0 0  0 1 0 0 0  0 0 1 0 0  0 0 0 19 -9" result="goo" />
                        <feComposite in="SourceGraphic" in2="goo" operator="atop"/>
                    </filter>
                </defs>
                <g class="plane-group">
                    <path class="paper-wing" d="M47.377,76.654V47.025c0-1.137,0.871-2.084,2.003-2.178l60.357-5.062c0.2-0.01,0.2-0.29,0-0.31l-60.356-5.052
                    c-1.133-0.095-2.004-1.042-2.004-2.179V2.615c0-1.878,1.922-3.142,3.646-2.399l87.37,37.662c1.54,0.664,1.54,2.848,0,3.512
                    l-87.37,37.662C49.299,79.796,47.377,78.531,47.377,76.654z"/>
                </g>
            </svg>
    <div class="paper-tail-div"></div>
  </div>
</div>