[css-values-4] 'lh' and 'rlh' unit need to explain how to convert to an absolute length

[dbaron]

The definitions of the lh unit and the rlh unit need to explain how to convert a line-height to an absolute length. In particular, this is interesting for normal, and it probably needs to say that it uses the first available font (which isn't the way normal works in layout).

(This came out of the discussion the WG is currently having.)

[frivoal]

This should be revisited once the definition of line-height itself is clarified, as being discussed in #1796

[frivoal]

Now that we have fixed #1796, this becomes much more simple to do.
For all values of line-height other than normal, the answer is now clearly non ambiguous, and for normal, I suggest the answer is the height that one would get for an empty line box, containing only the strut, which matches @dbaron 's suggestion to used only the first available font.

[css-meeting-bot]

The Working Group just discussed [css-values-4] 'lh' and 'rlh' unit need to explain how to convert to an absolute length, and agreed to the following resolutions:

• RESOLVED: Behave as strut

The full IRC log of that discussion

<dael> Topic: [css-values-4] 'lh' and 'rlh' unit need to explain how to convert to an absolute length
<dael> github: https://github.com//issues/1253
<dael> florian: We used to say they need to convert to absolute height. Was ambiguous. We fixed that. Now it's not ambig. There's not question how they should work for values other then normal. For normal we need to say something.
<fantasai> +1 to Florian's proposal
<dael> florian: reasonable?
<dael> Rossen_: Yes to me.
<dael> dbaron: Behavior as strut sounds good to me
<dael> RESOLVED: Behave as strut