Virginia DeBolt has written what is possibly the single most important tip for making websites readable — after good writing and an appropriate font, of course — and that’s line-height:
The CSS line-height property determines the distance or spacing between the lines of text on the page. In the print world, this property is called leading. …
Generally, you don’t want the lines to be too close together or too far apart. You want enough space between the lines to create maximum readability, or the appearance of ease of reading. You want an open and inviting look rather than a “dense” look, which discourages reading.
She shows some examples, and explains how to do line-height properly.
I must say, line-height is probably the thing that most affects me on websites. Dense text is highly likely to drive me away from even a well-written and well-presented site.
Dense text is tight and pressured and hard to read. Open text is relaxed and easy. It gives you room to ‘breathe’ while you read.
The only thing to watch out for is adding so much line-height that you can’t stay on track, but Virginia covers that too.
If you haven’t considered line-height before nip over to the tutorial and store the technique away in your coding toolbox.