Dewey Library - TP559.M53 M34 2019

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Create a simple CSS folded-corner effect without images or extra markup. It works well in all modern browsers and is best suited to designs with simple colour backgrounds.

Demo: Pure CSS folded-corner effect

Known support: Firefox 3.5+, Chrome 4+, Safari 4+, Opera 10+, IE 8+

This post is going to expand on the technique used to create the folded-corner effect that is part of the demo page for Multiple Backgrounds and Borders with CSS 2.1. As a starting point it will look to recreate the appearance of the note style used on the Yiibu‘s fantastic web site. Where Yiibu uses images, this will use pseudo-elements.

Nothing complicated. Any element will do and there’s no need for extra markup. It’s just a simple coloured box to start with. Browsers with no support for pseudo-elements, such as IE6 and IE7, will only be capable of displaying this.

Adding position:relative makes it possible to absolutely position the pseudo-element.

.note {
  position: relative;
  width: 30%;
  padding: 1em 1.5em;
  margin: 2em auto;
  color: #fff;
  background: #97C02F;
}

The folded-corner

The folded-corner is created from a pseudo-element that is positioned in the top corner of the box. The pseudo-element has no width or height but is given a thick border. Varying the size of the border will vary the size of the folded-corner.

In this example, the top and right borders are set to colours that match the background colour of the box’s parent. The left and bottom border are then given a slightly darker or lighter shade of the box’s background colour.

.note:before {
  content: "";
  position: absolute;
  top: 0;
  right: 0;
  border-width: 0 16px 16px 0;
  border-style: solid;
  border-color: #658E15 #fff;
}

This is all that’s needed to create a simple folded-corner effect like that found on Yiibu.

Firefox 3.0 doesn’t allow for the positioning of pseudo-elements. You can throw in a couple of extra styles to help tidy things up in that browser.

.note:before {
  ...
  display: block;
  width: 0;
}

Adding a subtle shadow

The appearance of a fold can be slightly enhanced by adding a box-shadow (for browsers that support it) to the pseudo-element. Setting overflow:hidden on the note itself will help hide parts of the shadow that would disrupt the folded-corner effect.

.note:before {
  ...
  -webkit-box-shadow: 0 1px 1px rgba(0,0,0,0.3), -1px 1px 1px rgba(0,0,0,0.2);
  -moz-box-shadow: 0 1px 1px rgba(0,0,0,0.3), -1px 1px 1px rgba(0,0,0,0.2);
  box-shadow: 0 1px 1px rgba(0,0,0,0.3), -1px 1px 1px rgba(0,0,0,0.2);
}

Rounded corners

It’s also relatively simple to make this work with rounded corners if desired. Unfortunately, every modern browser has some form of border-radius bug – including those using the non-prefixed property – which means a slight work around is needed.

Webkit browsers are the only browsers that can come anywhere close to rounding the corner of the pseudo-element if it only has 2 borders. Opera 11 and Firefox 3.6 make a mess of it. Opera 11 makes the biggest mess.

Using 4 borders avoids the problems in Opera 11 and Firefox 3.6. But it will trigger a bug in Safari 5 that leaves the diagonal looking a little jaggy. We can get around this problem by setting at least one border colour to be transparent.

When a background colour is applied to the pseudo-element it will show through the transparent border. Ideally, this approach would form the basis of the entire effect because we could reduce the amount of code needed. But Opera 11 will not show the background colour through the transparent borders unless a border-radius has been set.

.note-rounded:before {
  content: "";
  position: absolute;
  top: 0;
  right: 0;
  border-width: 8px;
  border-color: #fff #fff transparent transparent;
  background: #658E15;
  -moz-border-radius: 0 0 0 5px;
  border-radius: 0 0 0 5px;
  display: block;
  width: 0;
}

The CSS file on the demo page has more comments on the work arounds. Every browser has its own peculiarities when it comes to using border-radius or borders on elements with no width or height. This is the merely simplest solution I’ve found to deal with those browser inconsistencies.

The final code

This is all the CSS needed to create a simple folded-corner effect, with a subtle shadow, from a single HTML element. To include a variant with rounded corners, the “note” object can be extended with the modifications described previously.

.note {
  position: relative;
  width: 30%;
  padding: 1em 1.5em;
  margin: 2em auto;
  color: #fff;
  background: #97C02F;
  overflow: hidden;
}

.note:before {
  content: "";
  position: absolute;
  top: 0;
  right: 0;
  border-width: 0 16px 16px 0;
  border-style: solid;
  border-color: #fff #fff #658E15 #658E15;
  background: #658E15;
  -webkit-box-shadow: 0 1px 1px rgba(0,0,0,0.3), -1px 1px 1px rgba(0,0,0,0.2);
  -moz-box-shadow: 0 1px 1px rgba(0,0,0,0.3), -1px 1px 1px rgba(0,0,0,0.2);
  box-shadow: 0 1px 1px rgba(0,0,0,0.3), -1px 1px 1px rgba(0,0,0,0.2);
  /* Firefox 3.0 damage limitation */
  display: block; width: 0;
}

.note.rounded {
  -moz-border-radius: 5px 0 5px 5px;
  border-radius: 5px 0 5px 5px;
}

.note.rounded:before {
  border-width: 8px;
  border-color: #fff #fff transparent transparent;
  -moz-border-radius: 0 0 0 5px;
  border-radius: 0 0 0 5px;
}

The demo page shows the final effect, an example with rounded corners, and how different coloured notes are easy to create from this base.

This technique works less well when the element receiving the folded-corner effect is sitting on top of a background image rather than a simple background colour. However, the same limitation exists for image-based methods of creating this effect.

Internet Explorer 9–11 suffer from various bugs that prevent proper scaling of inline SVG’s. This is particularly problematic for SVG icons with variable widths. This is the canvas-based hack I’ve been using to work around the issue.

A popular way to use SVG icons is to generate a spritemap of SVG symbol‘s that you then reference from elsewhere in a document. Most articles on the topic assume your icon dimensions are uniformly square. Twitter’s SVG icons (crafted by @sofo) are variable width, to produce consistent horizontal whitespace around the vectors.

Most browsers will preserve the intrinsic aspect ratio of an SVG. Ideally, I want to set a common height for all the icons (e.g., 1em), and let the browser scale the width of each icon proportionally. This also makes it easy to resize icons in particular contexts – just change the height.

Unfortunately, IE 9–11 do not preserve the intrinsic aspect ratio of an inline SVG. The svg element will default to a width of 300px (the default for replaced content elements). This means it’s not easy to work with variable-width SVG icons. No amount of CSS hacking fixed the problem, so I looked elsewhere – and ended up using canvas.

canvas and aspect ratios

A canvas element – with height and width attributes set – will preserve its aspect ratio when one dimension is scaled. The example below sets a 3:1 aspect ratio.

<canvas height="1" width="3"></canvas>

You can then scale the canvas by changing either dimension in CSS.

canvas {
  display: block;
  height: 2rem;
}

Demo: proportional scaling of canvas.

Fixing SVG scaling in IE

This makes canvas useful for creating aspect ratios. Since IE doesn’t preserve the intrinsic aspect ratio of SVG icons, you can use canvas as a shim. A canvas of the correct aspect ratio provides a scalable frame. The svg can then be positioned to fill the space created by this frame.

The HTML is straightforward:

<div class="Icon" role="img" aria-label="Twitter">
  <canvas class="Icon-canvas" height="1" width="3"></canvas>
  <svg class="Icon-svg">
    <use fill="currentcolor" xlink:href="#icon-twitter"></use>
  </svg>
</div>

So is the CSS:

.Icon {
  display: inline-block;
  height: 1em; /* default icon height */
  position: relative;
  user-select: none;
}

.Icon-canvas {
  display: block;
  height: 100%;
  visibility: hidden;
}

.Icon-svg {
  height: 100%;
  left: 0;
  position: absolute;
  top: 0;
  width: 100%;
}

Setting the canvas height to 100% means it will scale based on the height of the component’s root element – just as SVG’s do in non-IE browsers. Changing the height of the Icon element scales the inner SVG icon while preserving its 3:1 aspect ratio.

Demo: proportional scaling of svg in IE.

Creating an Icon component

The hack is best added to (and eventually removed from) an existing icon component’s implementation.

If you’re generating and inlining an SVG spritemap, you will need to extract the height and width (usually from viewBox) of each of your icons during the build step. If you’re already using the gulp-svgstore plugin, it supports extracting metadata.

Those dimensions need to be set on the canvas element to produce the correct aspect ratio for a given icon.

Example React component (built with webpack):

import iconData from './lib/icons-data.json';
import React from 'react';
import './index.css';

class Icon extends React.Component {
  render() {
    const props = this.props;
    const height = iconData[props.name.height];
    const width = iconData[props.name.width];

    // React doesn't support namespaced attributes, so we have to set the
    // 'use' tag with innerHTML
    const useTag = `<use fill="currentcolor"
                      xlink:href="#icon-${props.name}">
                    </use>`;

    return (
      <span className="Icon">
        <canvas className="Icon-canvas"
          height={height}
          width={width}
        />
        <svg className="Icon-svg"
          dangerouslySetInnerHTML={{__html: useTag}}
          key={props.name}
        />
      </span>
    );
  }
}

export default Icon;

When I introduced this hack to a code base at Twitter, it had no impact on the the rest of the team or the rest of the code base – one of the many benefits of a component-based UI.

TrueNat machines will be used for the first time in the State for virus screening

The NCMC chaired by Cabinet Secretary Rajiv Gauba met on Friday to review the situation arising out of the gas leak

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