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.

The amount of research and testing that had to go into this demonstrates how fractured and broken web performance is right now. But, since it is, there's a useful snippet at the end of this article. Google Fonts is fast. Now it’s faster. Much faster.

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

With handy charts.

An excellent and highly educational series.