Icon Fonts: How do they get rendered by web pages?

Question

I understand that icon fonts are just fonts and that you can get the icons by just calling their classname, but how do icon fonts work?

I've tried checking the related icon font resources loaded in Chrome to see how icon fonts display icons (in comparison to general fonts) but I haven't been able to figure out how this happens.

I've also been unsuccessful in finding resources on how this "icon font technique" is done, even though there are loads of icon fonts available. There are also loads of resources showing how icon fonts can be integrated, but no one seems to be sharing or writing about how this is done!

Welby

Answer 1

Glyphicons are images and not a font. All the icons are found within a sprite image (also available as individual images) and they are added to the elements as positioned backround-images:

Actual font icons (FontAwesome, for instance) do involve downloading a specific font and make use of the content property, for instance:

@font-face {
    ...
    src: url('../font/fontawesome-webfont.eot?#iefix&v=3.0.1') format('embedded-opentype'),
         url('../font/fontawesome-webfont.woff?v=3.0.1') format('woff'),
         url('../font/fontawesome-webfont.ttf?v=3.0.1') format('truetype');
    ...
}

.icon-beer:before {
    content: "\f0fc";
}

As the content property isn't supported in older browsers, these also make use of images.

Here's an example of completely raw FontAwesome in use as a font, turning  ( - you may not be able to see this!) into an ambulance: http://jsfiddle.net/GWqcF/2

Salted

#12379653 - the answer says - its basically fonts – Welby

FontAwesome icons are fonts. I've even mentioned FontAwesome in my answer and gone on to say how they handle browsers which do not support their CSS method of adding the icons to the page. – Salted

Then how does font icons work ? - how can font be rendered as icons ? - with font icons - are the sprites downloaded ? – Welby

The icons are the font's characters. As a crude example: the letter "Z" could be designed to look like a suitcase, saved as a font and then used on a website. – Salted

Sorry that i changed the question - I was under the impression that font icons and Glyphicons are the same. I know how sprites work. Wanted to know how font icons worked. – Welby

So you're telling that all the icons are really just distorted characters? – Welby

Here's an example of completely raw FontAwesome in use, turning  into an ambulance: jsfiddle.net/GWqcF/2 – Salted

@VivekChandra yes that's right! :-) Icon fonts are just like any other font except that the characters are styled to look like icons. FontAwesome uses a range of characters reserved for "private use". – Salted

Awesome. Thanks for all the resources - will look into it, things are more clearer now. – Welby

My Browser does only load little Boxes with e.g. F0F9 inside of it. Why? – Roseola

@EdwardBlack did you remember the `` character? Have you loaded Font-Awesome correctly? – Salted

I opened the jsfiddle from you, and only little boxes are loaded – Roseola

@JamesDonnelly, Can you explain how material icons work. #45324077 – Ferguson

Urls in this jsfiddle are out of date, just replace them with actual (e.g. https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/fonts/fontawesome-webfont.eot and so for .woff and .ttf), you'll see image in the second string. – Incapacity

Does fontawesome download all the fonts upfront? – Fibrilla

Answer 2

If your question is how a CSS class can insert a specific character (that will be rendered as an icon in the special font), take a look at the source for FontAwesome:

.icon-glass:before { content: "\f000"; }
.icon-music:before { content: "\f001"; }
.icon-search:before { content: "\f002"; }
.icon-envelope:before { content: "\f003"; }
.icon-heart:before { content: "\f004"; }

So a CSS content directive is used to insert the character (which is from a special private-use reserved area of Unicode that does not mess up other readers).

Arbitrage

What does the slash f mean? – Guiana

The f is not part of it, it's just the hex number 15. The backslash starts the escape sequence for a hexadecimal codepoint. \f004 in CSS is like  in HTML. – Arbitrage

Answer 3

How webfont icons work?

Web-fonts icons work by using CSS to inject a specific glyph into the HTML using the content property. It then uses @font-face to load a dingbat webfont that styles the injected glyph. The upshot is that that injected glyph becomes the desired icon.

To begin, you’ll need a web-font file with the icons you need, either defined for particular ASCII characters (A, B, C, !, @, #, etc.) or in the Private Use Area of the Unicode font, which are spaces in the font that will not be used by specific characters in a Unicode encoded font.

Read more, how to create webfont icon at Responsive Webfont Icons.

Nary

Answer 4

The most popular answer says that Font Awesome uses images. From what I understood, it does not use images in the normal case.

Let's do some experiments to dive into details.

I used the following minimal code to test Font Awesome. You can try it yourself. Copy-paste it into a static HTML file and open the file in a browser.

<html lang="en">
  <head>   
    <link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.13.0/css/all.css"> 
  </head>
  
  

  <body>
  
      <i class="fas fa-desktop" ></i>
      <i class="fas fa-desktop" style="color:red"></i>
      <i class="fas fa-desktop" style="color:blue;font-style:italic"></i>
       <i class="fas fa-desktop" style="color:green;font-style:italic;font-size:40px"></i>

</body>
</html>

As you can see, I have used a Font Awesome desktop icon with style applied on top of it using the simple style="" attribute. Here is the result..

It responds to everything that a font can respond to. It responds to color, It responds to italics, and it responds to font size.

So to me, it does not look like a static image. Maybe it is SVG? Open Chrome network tab and refresh to load the static HTML file again. I see only these calls:

There is one font file: fa-solid-900.woff2 (web open font format). Download this file just for the experiment and put it within the same folder as our experimental HTML file, we will refer to it in next experiment.

Let's create a new HTML file without refering to Font Awesome CSS and just using the font file:

<html lang="en">
  <head>   
    
    
    <style>
    
    @font-face {
        font-family: "Font Awesome 5 Free";
        font-style: normal;
        font-weight: 900;
        font-display: block;
        src: url(fa-solid-900.woff2)
    }
    
    .my-custom-desktop-icon:before 
    {
      font-weight: 900;
      font-family: "Font Awesome 5 Free";
      content: "\f108";
    }
    
    </style>
    
  </head>
  
  

  <body>
  Custom Icon classes
  
      <span class="my-custom-desktop-icon" ></span>
      <i class="my-custom-desktop-icon" style="color:red"></i>
      <i class="my-custom-desktop-icon" style="color:blue;font-style:italic"></i>
       <i class="my-custom-desktop-icon" style="color:green;font-style:italic;font-size:40px"></i>

</body>
</html>

Result is the same:

I do not understand the full details of font creation yet. However, this is a high-level understanding. For each font icon class Font Awesome has specified a special font shape which it accesses by using a special character sequence. For desktop icon it is \f108

Font Awesome uses CSS classes with the selector :before for linking each font shape with a class name. Like the following...

.my-custom-desktop-icon:before 
        {
          font-weight: 900;
          font-family: "Font Awesome 5 Free";
          content: "\f108";
        }

If we look at the Font Awesome CSS file there are references to many other src for a font but I think the browser loads the first compatible file only. So for most browsers, image files are not required.

Maidel