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Using CSS to Replace Text with Images

Monday, 29 October 2007

Lots has been said about ways to replace text with images so that users with a graphical browser get a nice pretty logo, whilst search engines and screen readers get to see the text version. Most recently, Eric Enge has posted A Comprehensive Guide to Hidden Text & Search Engines over at SEOmoz. In general, I think it's a fair summary of the techniques I've encountered.

However, I was surprised to see the order of entries in the "may be OK" list. Firstly, I'd have expected sIFR to be top of the list — this is a widely used technique, and just replaces existing text with the same text in a different font. I prefer to do without Flash where possible, and this only works where you want to change the font rather than use a logo, but I can certainly see the draw here.

Secondly, I was also surprised to see the suggestion that is top of the list is to position the text off screen. I think this is a really bad idea, for accessibility reasons. When I only had a dial-up connection, I often used to browse with images turned off in order to reduce download times. If the text is positioned off screen, I would have just got a blank space. Even now, I often check websites with images turned off, because I think it is important. It is for this reason that my preferred technique is "Fahrner Image Replacement" (FIR). Whilst Eric says this is a no-no according to the Google Guidelines, I can't really see how — it's not deceptive in intent, and the text is seen by users without image support (or with images turned off) as well as the search engine bots. Also, given the quote from Susan Moskwa, it seems fine. Here's a quick summary of how it works:

Overlaying text with an image in CSS

The key to this technique is to have a nested SPAN with no content, position it over the text, and set a background image on it. If the background image loads, it hides the original text.

<h1 id="title"><span></span>Some Title Text</h1>

It is important to set the size of the enclosing tag to match the image, so that the hidden text doesn't leak out round the edges at large font sizes. The CSS is simple:

    position: relative;
    width: 200px;
    height: 100px;
    margin: 0px;
    padding: 0px;
    overflow: hidden;

#title span
    position: absolute;
    top: 0px;
    left: 0px;
    width: 200px;
    height: 100px;
    background-image: url(/images/title-image.png);

This simple technique works in all the major browsers, including Internet Explorer, and gracefully degrades. Obviously, you can't select text from the image, but you can generally select the hidden text (though it's hard to see what you're doing), and copying the whole page will include the hidden text. Check it out — how does the title above ("Overlaying text with an image in CSS") appear in your browser?

Update: It has been pointed out in a comment on the linked SEOmoz article by bjornjohansen that you need to be aware of the potential for browsers with a different font size. This is definitely important — that's why we specify the exact dimensions for the enclosing element, and use overflow: hidden to avoid overhang. It's also important to ensure that the raw text (without the image) fits the specified space when rendered in at least one font size larger than "normal", so that people who use larger fonts can still read it with images disabled, without getting the text clipped.

Update: In another comment over on the SEOmoz article, MarioFr suggested that for headings the A tag could be used instead of SPAN — since empty A tags can be used as a link target in the heading, it works as a suitable replacement. I've changed the heading above to use an A tag for both purposes as an example.

Posted by Anthony Williams
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