Image Replacement in CSS3

Image replacement techniques are used on many websites today to replace text, such as a header, with an image of the text rendered in a fancy font. Methods range from image tags to background images with massing negative indentation, each with their own disadvantage.

The CSSWG has resolved to allow the content property on all elements in CSS3. This property can take a <uri>, creating a replaced element. In CSS3, content takes a comma-separated list, so that fallbacks can be specified. Here's an example:

  /* Use image, failing that use element's content. */
  H1 { content: url(sparkly_heading_text.png), contents }

Interaction with Fonts

We recognize, however, that the likely set of preferences for such headings would be:

  1. Use the preferred font if it is installed on the system.
  2. Otherwise download the font and use it, if possible.
  3. Otherwise display the image in place of the contents, if possible.
  4. Otherwise display the contents in an available font.

The font parts (preferred font, downloaded font, available font) can be done in CSS2 with font-family and @font-face, but incorporating image replacement into the list requires something more.

At the May 2006 face-to-face meeting, the CSSWG accepted a proposal from Ian to add a require-font function to the content property. The function would not create any content, but would trigger the next fallback if the fonts in its argument list (whether pre-installed or downloaded via @font-face) were not available for use. Multiple require-font functions would be allowed.

The consensus on syntax was:

 Syntax:
    require-font
    require-font(<string>)
    require-font(<family-name>)
 Edge cases to cover:
    require-font()
    require-font(generic-name) (e.g. require-font(serif))

The require-font keyword would automatically take the first font in font-family as its implied argument. This is not merely syntactic sugar for the author, but also causes a user's font override, if any, to become the required font: in typical usage, this would disable the image replacement fallback and display the contents in the user's selected font.

The require-font keyword or function listed by itself without any other associated content value would imply contents (i.e. the usual content of the element).

Unresolved Details

One proposal for require-font() was to make it the same as require-font. The other, which avoids creating an alias (which would be a pain for DOM-based editors), would interpret it as requiring the font named “” [the empty string], effectively always triggering the fallback. The third possibility would be to make it invalid, causing it to invalidate the entire property declaration.

It was assumed that require-font(generic-name) would always be true. However this needs to be specified: figuring out require-font(serif) on a system with only sans-serif fonts is not obvious.

The spec should perhaps specify that require-font() is always successful in non-visual media.

Would require-font(Arial, Verdana) require both, require at least one, or be syntactically invalid? (Since require-font(Arial) require-font(Verdana) does the && operation, having the comma mean || would be the most useful.)

Examples

A basic example from Ian, demonstrating a range of fallback possibilities.

<h1>Hello World</h1>

…with one of:

h1 { content: "Hello"; }
h1 { content: url(images/hello.png), "Hello"; }
h1 { content: url(images/hello.png); }
h1 { font-family: Zapfino, cursive;
     content: require-font("Zapfino") "Hello"; }
h1 { font-family: Zapfino, cursive;
     content: require-font("Zapfino") contents; }
h1 { font-family: Zapfino, cursive;
     content: require-font("Zapfino") }
h1 { font-family: Zapfino, cursive;
     content: require-font("Zapfino") contents, url(images/hello.png); }
h1 { font-family: Zapfino, Wingdings, Arial;
     content: require-font("Zapfino") require-font("Wingdings") contents, url(images/hello.png); }

Another example from Bert, demonstrating the advantage of separating the method of specifying a font (font-family) from requiring it (require-font on content):

<h1>This is <span>huge!</span></h1>
h1 span { font-family: biggy }
h1      { font-family: Times New Roman }
h1      { content: require-font(biggy) contents, url(replacement.png) }

The contents keyword is implied and the last line can therefore be written:

h1      { content: require-font(biggy), url(replacement.png) }

Expected Spec

Since the Generated Content module is not expected to advance at all for quite some time, this syntax is likely to be incorporated into the Fonts module.

 
ideas/image-replacement.txt · Last modified: 2014/12/09 15:48 (external edit)
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