> i.e. horizontal centering is easy because that's a natural thing to want to do in a document, but vertical centering seems to make less sense and is thus corr

> i.e. horizontal centering is easy because that's a natural thing to want to do in a document, but vertical centering seems to make less sense and is thus correspondingly more difficult. It’s because text wraps horizontally and flows from top to bottom. The width is constrained by the window but the height is defined by the contents of the document. This means that if you are trying to vertically centre something in the document, you need to know the size of the document before you can render its contents, but you need to render its contents to know the size of the document. There’s various ways around this but they all have different trade-offs, hence multiple solutions to the problem of vertical centring in CSS.