Every developer runs into some of those annoying little quirks in CSS. They can leave you scratching your head for days, wondering why you element ...
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You can also use flexbox on the parent element:
By altering line-height, you're only getting vertical alignment as a side-effect, so it may be better to apply it to the parent instead.
The preferred way to use 'line-height' is with integer, lika 'line-height: 1;'. This number multiplied by the element's font-size.
This wouldn't have the effect of vertically centering the text in its parent though. This is a particular technique, otherwise I agree that using unit-less values is more useful.