How Can We Reliably Detect Which Font is Actually Rendered on a Web Page?

Detecting Specific Fonts in Web Pages

Determining which font is rendered on a web page is crucial for ensuring optimal user experience. Consider a CSS rule that defines multiple fonts:

body {
    font-family: Calibri, Trebuchet MS, Helvetica, sans-serif;
}

How can we detect which of these fonts is actually displayed on the user's browser?

The reason for this determination is critical because specific fonts may contain unique glyphs not available elsewhere. Hence, If the user lacks the desired font, it's preferable to provide a download link. However, currently, the download link is displayed to all users, even those with the correct font installed.

One ingenious but potentially unreliable method involves creating an element with a specific font and text. If the browser lacks the specified font, it inherits the font from its parent element. By measuring the width of the rendered text and comparing it to the expected width for both the desired and inherited fonts, the presence of the desired font can be determined. This technique is not suitable for monospaced fonts, however.

For further insights on this topic, refer to the following reference:

• Javascript/CSS Font Detector (ajaxian.com; 12 Mar 2007)

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