Line length and readability
The width of a paragraph changes how easily people read it. The sweet spot is narrower than most layouts allow.
The measure
Typographers call the width of a line of text its measure. For centuries, book designers have kept it between roughly 45 and 75 characters, and the web has rediscovered the same range. Lines much longer than that make it hard for the eye to find the start of the next line. Lines much shorter break the rhythm of reading with constant returns.
Why wide screens make it worse
A paragraph that fills the width of a large monitor can easily run past 150 characters per line. Readers lose their place, skim more, and remember less. The fix is simple: constrain the width of text containers, not the page. In CSS, max-width: 65ch sets a limit based on the width of the zero character in the current font, which tracks the font size automatically.
Line height follows measure
Longer lines need more space between them so the eye can track back to the correct starting point. Shorter lines can be set tighter. A line height around 1.5 suits body text at a comfortable measure. Headlines, which are short and large, usually look better closer to 1.1 or 1.2.
Font size
On screens, body text below 16 pixels becomes tiring for many readers, especially on phones held at arm's length. Many reading-focused sites now set body text at 18 to 20 pixels on larger screens. Use relative units so that a reader's browser preference still has an effect.
Paragraph spacing
Space between paragraphs should be clearly larger than the space between lines, so that paragraphs read as distinct units. A margin equal to about one line height is a reliable starting point.
Headings that group
A heading belongs to the content below it, so it should sit closer to that content than to the paragraph above. Many default stylesheets set equal margins above and below headings, which makes them float ambiguously between sections. Increase the space above and reduce the space below.
Responsive measure
On a narrow phone, the screen width already limits the measure, and the main concern is comfortable side padding. On tablets and desktops, the max-width takes over. Check the layout at intermediate widths too, where a sidebar may appear and squeeze the text column into something too narrow.
Test with real text
Lorem ipsum hides problems because its word lengths and rhythm differ from real prose. Set a long article in your template and actually read it. If you find yourself losing your place or squinting, the reader will too.