How to detect which one of the defined font was used in a web page?

A technique that works is to look at the computed style of the element. This is supported in Opera and Firefox (and I recon in safari, but haven't tested). IE (7 at least), provides a method to get a style, but it seems to be whatever was in the stylesheet, not the computed style. More details on quirksmode: Get Styles

Here's a simple function to grab the font used in an element:

/**
 * Get the font used for a given element
 * @argument {HTMLElement} the element to check font for
 * @returns {string} The name of the used font or null if font could not be detected
 */
function getFontForElement(ele) {
    if (ele.currentStyle) { // sort of, but not really, works in IE
        return ele.currentStyle["fontFamily"];
    } else if (document.defaultView) { // works in Opera and FF
        return document.defaultView.getComputedStyle(ele,null).getPropertyValue("font-family");
    } else {
        return null;
    }
}

If the CSS rule for this was:

#fonttester {
    font-family: sans-serif, arial, helvetica;
}

Then it should return helvetica if that is installed, if not, arial, and lastly, the name of the system default sans-serif font. Note that the ordering of fonts in your CSS declaration is significant.

An interesting hack you could also try is to create lots of hidden elements with lots of different fonts to try to detect which fonts are installed on a machine. I'm sure someone could make a nifty font statistics gathering page with this technique.

I've seen it done in a kind of iffy, but pretty reliable way. Basically, an element is set to use a specific font and a string is set to that element. If the font set for the element does not exist, it takes the font of the parent element. So, what they do is measure the width of the rendered string. If it matches what they expected for the desired font as opposed to the derived font, it's present. This won't work for monospaced fonts.

Here's where it came from: Javascript/CSS Font Detector (ajaxian.com; 12 Mar 2007)

@pat Actually, Safari does not give the font used, Safari instead always returns the first font in the stack regardless of whether it is installed, at least in my experience.

font-family: "my fake font", helvetica, san-serif;

Assuming Helvetica is the one installed/used, you'll get:

• "my fake font" in Safari (and I believe other webkit browsers).
• "my fake font, helvetica, san-serif" in Gecko browsers and IE.
• "helvetica" in Opera 9, though I read that they are changing this in Opera 10 to match Gecko.

I took a pass at this problem and created Font Unstack, which tests each font in a stack and returns the first installed one only. It uses the trick that @MojoFilter mentions, but only returns the first one if multiple are installed. Though it does suffer from the weakness that @tlrobinson mentions (Windows will substitute Arial for Helvetica silently and report that Helvetica is installed), it otherwise works well.

I wrote a simple JavaScript tool that you can use it to check if a font is installed or not.
It uses simple technique and should be correct most of the time.

jFont Checker on github