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If you're using a web font, you're bound to see a flash of unstyled text (or FOUC), between the initial render of your websafe font and the webfont that you've chosen. This usually results in a jarring shift in layout, due to sizing discrepancies between the two fonts. To minimize this discrepancy, you can try to match the fallback font and the intended webfont’s x-heights and widths [1]. This tool helps you do exactly that.

Intelligently choosing the graceful degradation (size- and style-wise) of your web fonts.

With handy charts.

An excellent and highly educational series.