How do CSS triangles work?

Question

There're plenty of different CSS shapes over at CSS Tricks - Shapes of CSS and I'm particularly puzzled with a triangle:

#triangle-up {
  width: 0;
  height: 0;
  border-left: 50px solid transparent;
  border-right: 50px solid transparent;
  border-bottom: 100px solid red;
}

<div id="triangle-up"></div>

How and why does it work?

Answer 1

OK, this triangle will get created because of the way that borders of the elements work together in HTML and CSS...

As we usually use 1 or 2px borders, we never notice that borders make a 45° angles to each others with the same width and if the width changes, the angle degree get changed as well, run the CSS code I created below:

.triangle {
  width: 100px;
  height: 100px;
  border-left: 50px solid black;
  border-right: 50px solid black;
  border-bottom: 100px solid red;
}

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

Then in the next step, we don't have any width or height, something like this:

.triangle {
  width: 0;
  height: 0;
  border-left: 50px solid black;
  border-right: 50px solid black;
  border-bottom: 100px solid red;
}

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

And now we make the left and right borders invisible to make our desirable triangle as below:

.triangle {
  width: 0;
  height: 0;
  border-left: 50px solid transparent;
  border-right: 50px solid transparent;
  border-bottom: 100px solid red;
}

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

If you not willing to run the snippet to see the steps, I've created an image sequence to have a look at all steps in one image:

Answer 2

Here is an animation in JSFiddle I created for demonstration.

Also see snippet below.

This is an Animated GIF made from a Screencast

transforms = [
         {'border-left-width'   :'30', 'margin-left': '70'},
         {'border-bottom-width' :'80'},
         {'border-right-width'  :'30'},
         {'border-top-width'    :'0', 'margin-top': '70'},
         {'width'               :'0'},
         {'height'              :'0', 'margin-top': '120'},
         {'borderLeftColor'     :'transparent'},
         {'borderRightColor'    :'transparent'}
];


$('#a').click(function() {$('.border').trigger("click");});
(function($) {
    var duration = 1000
    $('.border').click(function() {
		  for ( var i=0; i < transforms.length; i++ ) {
        $(this)
         .animate(transforms[i], duration)
		  }
    }).end()
}(jQuery))

.border {
    margin: 20px 50px;
    width: 50px;
    height: 50px;
    border-width: 50px;
    border-style: solid;
    border-top-color: green;
    border-right-color: yellow;
    border-bottom-color: red;
    border-left-color: blue;
    cursor: pointer
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="https://code.jquery.com/color/jquery.color-2.1.2.min.js"></script>
Click it!<br>
<div class="border"></div>

---

Random version

/**
 * Randomize array element order in-place.
 * Using Durstenfeld shuffle algorithm.
 */
function shuffleArray(array) {
    for (var i = array.length - 1; i > 0; i--) {
        var j = Math.floor(Math.random() * (i + 1));
        var temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    }
    return array;
}

transforms = [
         {'border-left-width'   :'30', 'margin-left': '70'},
         {'border-bottom-width' :'80'},
         {'border-right-width'  :'30'},
         {'border-top-width'    :'0', 'margin-top': '70'},
         {'width'               :'0'},
         {'height'              :'0'},
         {'borderLeftColor'     :'transparent'},
         {'borderRightColor'    :'transparent'}
];
transforms = shuffleArray(transforms)



$('#a').click(function() {$('.border').trigger("click");});
(function($) {
    var duration = 1000
    $('.border').click(function() {
		  for ( var i=0; i < transforms.length; i++ ) {
        $(this)
         .animate(transforms[i], duration)
		  }
    }).end()
}(jQuery))

.border {
    margin: 50px;
    width: 50px;
    height: 50px;
    border-width: 50px;
    border-style: solid;
    border-top-color: green;
    border-right-color: yellow;
    border-bottom-color: red;
    border-left-color: blue;
    cursor: pointer
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="https://code.jquery.com/color/jquery.color-2.1.2.min.js"></script>
Click it!<br>
<div class="border"></div>

---

All at once version

$('#a').click(function() {$('.border').trigger("click");});
(function($) {
    var duration = 1000
    $('.border').click(function() {
        $(this)
         .animate({'border-top-width': 0            ,
         					 'border-left-width': 30          ,
         					 'border-right-width': 30         ,
         					 'border-bottom-width': 80        ,
         					 'width': 0                       ,
         					 'height': 0                      ,
                   'margin-left': 100,
                   'margin-top': 150,
         					 'borderTopColor': 'transparent',
         					 'borderRightColor': 'transparent',
         					 'borderLeftColor':  'transparent'}, duration)
    }).end()
}(jQuery))

.border {
    margin: 50px;
    width: 50px;
    height: 50px;
    border-width: 50px;
    border-style: solid;
    border-top-color: green;
    border-right-color: yellow;
    border-bottom-color: red;
    border-left-color: blue;
    cursor: pointer
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="https://code.jquery.com/color/jquery.color-2.1.2.min.js"></script>
Click it!<br>
<div class="border"></div>

Answer 3

Taking it one step further, using css based on this I added arrows to my back and next buttons (yes I know its not 100% cross-browser, but slick none the less).

.triangle {
  width: 0;
  height: 0;
  border-left: 50px solid transparent;
  border-right: 50px solid transparent;
  border-bottom: 100px solid red;
  margin:20px auto;
}

.triangle-down {
  border-bottom:none;
  border-top: 100px solid red;
}

.triangle-left {
  border-left:none;
  border-right: 100px solid red;
  border-bottom: 50px solid transparent;
  border-top: 50px solid transparent;
}

.triangle-right {
  border-right:none;
  border-left: 100px solid red;
  border-bottom: 50px solid transparent;
  border-top: 50px solid transparent;
}

.triangle-after:after {
  width: 0;
  height: 0;
  border-left: 5px solid transparent;
  border-right: 5px solid transparent;
  border-bottom: 5px solid red;
  margin:0 5px;
  content:"";
  display:inline-block;
}

.triangle-after-right:after {
  border-right:none;
  border-left: 5px solid blue;
  border-bottom: 5px solid transparent;
  border-top: 5px solid transparent;

}

.triangle-before:before {
  width: 0;
  height: 0;
  border-left: 5px solid transparent;
  border-right: 5px solid transparent;
  border-bottom: 5px solid blue;
  margin:0 5px;
  content:"";
  display:inline-block;
}

.triangle-before-left:before {
  border-left:none;
  border-right: 5px solid blue;
  border-bottom: 5px solid transparent;
  border-top: 5px solid transparent;

}

<div class="triangle"></div>
<div class="triangle triangle-down"></div>
<div class="triangle triangle-left"></div>
<div class="triangle triangle-right"></div>

<a class="triangle-before triangle-before-left" href="#">Back</a>
<a class="triangle-after triangle-after-right" href="#">Next</a>

Answer 4

The borders use an angled edge where they intersect (45° angle with equal width borders, but changing the border widths can skew the angle).

jsFiddle.

By hiding certain borders, you can get the triangle effect (as you can see above by making the different portions different colours). transparent is often used as an edge colour to achieve the triangle shape.

Answer 5

Others have already explained this well. Let me give you an animation which will explain this quickly: http://codepen.io/chriscoyier/pen/lotjh

Here is some code for you to play with and learn the concepts.

HTML:

<html>
  <body>
    <div id="border-demo">
    </div>
  </body>
</html>

CSS:

/*border-width is border thickness*/
#border-demo {
    background: gray;
    border-color: yellow blue red green;/*top right bottom left*/
    border-style: solid;
    border-width: 25px 25px 25px 25px;/*top right bottom left*/
    height: 50px;
    width: 50px;
}

Play with this and see what happens. Set height and width to zero. Then remove top border and make left and right transparent, or just look at the code below to make a css triangle:

#border-demo {
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 100px solid blue;
}