https://drafts.csswg.org/css-values/#lh says:

Equal to the computed value of the line-height property of the element on which it is used, converting normal to an absolute length by using only the metrics of the first available font.

But the computed value of line-height depends on other properties at least in some browsers, so this suddenly becomes really tricky.

For example, <select>'s line-height computed value for comboboxes changes in WebKit and Blink to normal if the select is themed (based on the -webkit-appearance) property:

https://crisal.io/tmp/select.html

I'm considering doing the same in Firefox in https://bugzilla.mozilla.org/show_bug.cgi?id=1501908.

In any case, the used value does definitely depend on theming of comboboxes, for example, across all browsers (see above).

How are these units supposed to work in this case? We definitely don't want values to depend on used values. We should probably specify how these adjustments work and make them work across browsers before anyone implements these units.

cc @lilles @FremyCompany @zcorpan