Creating responsive triangles with CSS

Question

I was trying to create triangles in CSS for a responsive site today and couldn't find a good example on stackoverflow, so here's how I did it.

Answer 1

Reponsive triangles can be achieved with just CSS by taking advantage of padding being calculated against parent’s width to cover a big fixed-width triangle. To create an up-pointing triangle with 100% width:

.triangle-up {
    width: 50%;
    height: 0;
    padding-left:50%;
    padding-bottom: 50%;
    overflow: hidden;
}
.triangle-up div {
    width: 0;
    height: 0;
    margin-left:-500px;
    border-left: 500px solid transparent;
    border-right: 500px solid transparent;
    border-bottom: 500px solid green;
}

Or using pseudoelements and just one div:

.triangle-up {
    width: 50%;
    height: 0;    
    padding-left:50%;
    padding-bottom: 50%;
    overflow: hidden;
}
.triangle-up:after {
    content: "";
    display: block;
    width: 0;
    height: 0;
    margin-left:-500px;
    border-left: 500px solid transparent;
    border-right: 500px solid transparent;
    border-bottom: 500px solid #959595;
}

Here's a fiddle. For the full explanation on how these work and the down, left and right pointing triangle snippets see my article on Pure CSS responsive triangles. The CSS given is for a triangle with base-height ratio of 2. Trying to change the triangle's proportions without knowing how these triangles fake responsiveness may be complicated.

Answer 2

Making angular shapes responsive is a little tricky because you can't use percentages as border values in your CSS, so I wrote a couple functions to calculate the page width and resize a triangle accordingly. The first calculates the size on loading the page, the second recalculates the size as the page width changes.

CSS:

.triangle {
    width: 0;
    height: 0;
    border-top: 50px solid rgba(255, 255, 0, 1);
    border-right: 100px solid transparent;
}

HTML:

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

JS:

$(document).ready(function () {
    var windowWidth = $(window).width();
    $(".triangle").css({
        "border-top": windowWidth / 2 + 'px solid rgba(255, 255, 0, 1)'
    });
    $(".triangle").css({
        "border-right": windowWidth / 1.5 + 'px solid transparent'
    });
});

$(window).resize(function () {
    var windowWidthR = $(window).width();
    $(".triangle").css({
        "border-top": windowWidthR / 2 + 'px solid rgba(255, 255, 0, 1)'
    });
    $(".triangle").css({
        "border-right": windowWidthR / 1.5 + 'px solid transparent'
    });
});

Here's a jsFiddle - http://jsfiddle.net/craigcannon/58dVS/17/