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The following sample code creates a workbook, adds some text to cell B4 of the first worksheet, and sets its font to an unknown/non‑existing font. Then it saves the workbook in HTML by setting different default font names such as Courier New, Arial, Times New Roman, etc.
The screenshot shows the effect of setting different default font names via the HtmlSaveOptions.default_font_name property.

The code generates the output HTML file with Courier New, the output HTML with Arial, and the output HTML file with Times New Roman.
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