In CSS, you can specify line-height of text that span multiple lines. Though you can specify this in pixels or other units, you should specify them without any units so it becomes a fraction or percentage of font size.
Example:
If you have font size set to 12 px and your line height is set to 18 px, you can convert your line height to 1.5 (18px/12px = 1.5). This way, if you font size changes or if you use a different font, the line height will adjust proportionally.