First-line indent

A first-line indent is the oldest way of saying “a new paragraph starts here”. It costs no vertical space, survives any measure, and stays invisible until it is needed: the eye finds the notch without ever being asked to look for one. In CSS it is a single property, text-indent, set on the block rather than on the line, because which words end up on that first line is whatever the wrapping happens to produce. Write it in ems, since the indent is a measurement of the type rather than of the page, and it should grow when the type does.

The rule most implementations miss is that the first paragraph after a heading, a rule, or a figure is set flush. An indent marks a break from the paragraph above, and after a heading there is no paragraph above, so the notch announces a boundary that does not exist. The fix is the selector rather than the value: p + p { text-indent: 2em } indents only a paragraph that actually follows one.

It is an alternative to space between paragraphs, not a partner. A blank line marks the same boundary, so setting both marks it twice, and the indent stops reading as a signal and starts reading as a ragged left edge. Space suits screens and short blocks, where a reader is scanning for entry points; the indent suits continuous prose, where nobody wants the column chopped into slabs. On size, one em is the usual floor and Practical Typography puts the useful range at one to four times the point size. Less than an em does not read as deliberate, and much more starts reading as a list.

A hanging indent is the same measurement pointed the other way: a first-line indent pushes the opening line in and leaves the rest flush, while a hanging indent leaves the opening line flush and pushes every line after it in. They answer different questions, which is why they almost never compete: the first says “a new paragraph” inside running prose, the second says “a new entry” in a list you look things up in. A drop cap is the loud version of the same signal, and it belongs to exactly the opening paragraph that takes no indent at all.