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Going the extra mile

Take a look at the following two circles. Which would you say is correctly centered?

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The second one looks better, doesn't it? And yet, the first one is the technically correct answer.

In CSS, everything is boxes. The number "1" produces a box around it, and that box is perfectly centered.

The one on the right has been shifted, so that its stem aligns with the vertical axis:

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This is known as optical alignment. It's aligned based on human perception, not based on the absolute mathematical distance between boxes.

Admittedly, this is a really small detail, and it becomes impractical if the numbers are data-driven. But it's a good example of how mathematical centering doesn't always feel right. Sometimes, we need to make small tweak so that our eyes believe that things are properly aligned.

I frequently find myself needing to make slight shifts like this. Let's look at another example. Toggle between the two options, and see which one feels better to you:

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If we use the element inspector, we see that the content box includes a few pixels of dead space before the left edge of the typography:

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Happily, the kerning that causes this gap does seem to be consistent across browser rendering engines, and so we can safely shift things by a few pixels for universally-improved alignment.

To be clear, I know that this is super subtle. You can achieve really sharp designs without doing this optical-alignment stuff. We're in "last 5%" territory. But it does make a difference, and for mission-critical pages (homepage, high-traffic landing pages), it can be worth spending a few minutes on these tweaks!

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