Line length and line height

What spoils long reading most isn't the typeface, it's line length. The longer the line, the harder it is for the eye to find the start of the next line when it jumps back.

A comfortable length is between 45 and 75 characters. The ch unit measures by character width directly, so you can write max-width: 65ch.

Line height is written without a unit. line-height: 1.7 means the line is 1.7 times the font size, whatever that size is, so it scales with each element's font. The larger the font, the less line height it needs, which is why headings get a smaller line height than paragraphs.