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.

‘There are some particular findings that are pivotal to issues such as readership and readability and comprehension, which is really what body copy is all about. If people won’t read it, or if they can’t read it or understand it, then what’s the point of having it?’

With handy charts.

An excellent and highly educational series.