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[css-values] Comments on Serialization of calc

From: Francois Remy <frremy@microsoft.com>
Date: Wed, 17 Feb 2016 19:39:56 +0000
To: CSS WG <www-style@w3.org>
Message-ID: <BN3PR0301MB0883A0E03F930F8BFFCFC89AD6AE0@BN3PR0301MB0883.namprd03.prod.outlook.com>
Hi Tab, Fantasai,

Here are a few comments regarding the "calc" serialization section I just reviewed:

----[0] Thanks for looking into this, you rock, as always!

----[1] I am not confident with the "compatible units" notion, mostly because it isn't defined anywhere (except by example). It is not clear, for instance, what calc(100vw - 20px) resolves to at specified time. Technically, the viewport width does not depend on the element this value is applied on, but at the same time it may vary which would somehow require to keep the values separate to provide the right result over time. It is also not clear to me what happens if we have calc(1cm + 1mm) vs calc(1mm + 1cm). The units are compatible, but which one should we keep? Does order matter?

An alternative approach I would like to see investigated would be the following one: 
(a) define a canonical unit per type (length:px,time:s,...) or an ordered list of canonical units if needed
(b) replace step 1 by something like "for each component of the sum, if it would possible to do so permanently, convert it into the canonical unit of the calc expression (specified values cannot be converted in a way that would make the conversion be invalid if the environment changes, computed values can)"

This is a breaking change over your current approach which preserves 0vh for instance, while it would be converted to 0px in this case. We could add some additional condition that says that the validity of conversion is considered independently of the number put in front of the unit, which prevents 0 being an exception).

----[2] I have no opinion on the clamping of values at step 2 of the algorithm you want to add. For me it looks fine to drop this step altogether and just serialize "calc(<value>)" whatever <value> is, but I understand some browsers might want to simplify it as soon as possible in order to save memory. This is such an edge case I doubt it is really worth it, though.

Hope this helps,
Received on Wednesday, 17 February 2016 19:40:26 UTC

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