Different font families at the exact same size have different x-heights, and so different aspect ratios.
Many web designers use font stacks to allow for differences in their customers’ computers, but most never think about what can happen if their first choice font isn’t available. In fact, I bet if I did a survey, I would find that most people don’t even test their pages in the second or third font in their stack, they just put them in there and move on to the next task. But this can be a problem. Even though you are using fonts at exactly the same size, they can take up more or less space and be more or less legible depending upon how the font is constructed. Luckily there is a CSS3 font property that can help fix this problem:
font-size-adjust. Learn how to use
font-size-adjust to correct how your pages look even if your first choice (and tested) font isn’t available.
Read the full article: Using the CSS3 font-size-adjust Property to Set Your Font Sizes
Understanding Font Aspect Ratios and Font Sizes
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