Using a relative unit such as EM is a great way to maintain the vertical rhythm of a web page when a user resizes text in their browser. Constantly reaching for a calculator to compute the correct EM value to use every time you need to is also a great way to drive yourself mad. Let’s say you have an H3 that’s 24 pixels. It doesn’t matter how the font-size is set (pixels, EMs, percentages, etc). You can check the actual font-size in pixels using Firebug by checking “Show Computed Style” in the Options menu in the upper right corner of the CSS/Style panel while viewing in HTML mode. You’ve checked the design comp and measured exactly 9 pixels of bottom margin under the H3 until the…
