Actually, you can achieve this pretty easy. Simply specify the line height as a number:
<p style="line-height:1.5">
<span style="font-size:12pt">The quick brown fox jumps over the lazy dog.</span><br />
<span style="font-size:24pt">The quick brown fox jumps over the lazy dog.</span>
</p>
The difference between number and percentage in the context of the line-height CSS property is that the number value is inherited by the descendant elements, but the percentage value is first computed for the current element using its font size and then this computed value is inherited by the descendant elements.
For more information about the line-height property, which indeed is far more complex than it looks like at first glance, I recommend you take a look at this online presentation.