I think there are almost other answers, but the linear height is slightly different. The way I think about this is that the length of the string is the amount of center space. Therefore, if your line-height is 50 pixels, the space above it will be 25 pixels, and 25 times the space below the middle of the letter. This makes the 50px line high.
Thus, to make the space between them, I would use margin-bottom and set the line height to what it looks like as it goes down to the top and bottom of the letter (possibly depending on the font you are using). In my example below, I set the line height to .7 (you can see how it goes down to the actual baseline and the top height with a red border. Then I gave the margin value with rem units so that it is relative to the original page font size, not div itself with a unique font size.
body { font-size: 24px; } div { line-height: .7; margin-bottom: 1rem; border: 1px solid red; }
<div style="font-size: 200%">A</div> <div style="font-size: 100%">B C</div> <div style="font-size: 50%">DE F</div> <div style="font-size: 25%">GH I</div>
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