CSS: centering things
https://www.w3.org/Style/Examples/007/center.en.html
A common task for CSS is to center text or images. In fact, there are three kinds of centering: Centering lines of text; Centering a block of text or an image; Centering a block or an image vertically. In recent implementations of CSS you can also use features from level 3, which allows centering absolutely positioned elements:
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