The 'lh' unit adds support for expressing CSS lengths relative to the line-height. That is typically useful for specifying the height of an element to fit a whole number of lines. Example: textarea { height: 5lh }
There is a wish/need to use the lh unit to size textareas to fit a given number of lines of text. See [1]. [1] https://github.com/w3c/csswg-drafts/issues/7542