Re: [css-values][css-writing-modes] ch and ic units

On Thu, Mar 10, 2016 at 6:36 AM, fantasai <fantasai.lists@inkedblade.net>
wrote:

> On 03/09/2016 08:39 AM, Koji Ishii wrote:
>
>> On Wed, Mar 9, 2016 at 4:55 PM, Florian Rivoal <florian@rivoal.net
>> <mailto:florian@rivoal.net>> wrote:
>>
>>     I prefer "ch" always be the "width of "0" glyph, or 0.5em" as what
>>> authors lose looks more than what authors get.
>>>
>>
>>     Why is the width of the 0 character interesting if text is set
>> upright? If it is mixed or sideways, I agree width is the
>>     right measure, but for upright I don't see it.
>>
>>
>> For me, "ch" is an approximate unit that represents approximate
>> average width of Latin characters, and I expect it to be
>> 60-80% of "em", similar to what "en" does in non-CSS world.
>> It doesn't guarantee to fit any specific number of characters in
>> horizontal flow, so I don't expect it in vertical flow either.
>>
>
> Several points:
>   * We have an en unit. It does not change per writing mode.
>

Where is it defined? I can't find in CSS Values[1]?


>   * ch is an exact character count in monospace fonts
>

That's true, but it's also true for me that it serves similar effects as
"en". I believe it's so to most authors.


>   * ch is also an exact character count for tabular numbers
>     in proportional fonts.
>

True if the width of "0" does not change by the "tnum" feature.


>   * The use case for ch is an exact (for monospace fonts) or
>     approximate (for proportional fonts) sizing function
>     with respect to the number of characters that can fit on
>     a line. Its primary use case is <textarea>, <input>, and
>     <pre>. Ignoring the effects of text-orientation here would
>     make it fail at its primary use case.
>

Doing so doesn't solve what you want to solve, since it only solves for
digits.


> Fwiw, the ch unit is already defined to be writing-mode sensitive,
> as the spec very intentionally uses the term "advance measure" and
> not "advance width".
>

I wasn't aware of that, if so, I'd request to change. This is too bad for
authors, and makes things unnecessarily complex.


> This was probably clearer when Writing Modes used the term "measure"
> to refer to inline sizes, so some clarification might help. However,
> the spec is nontheless unambiguous here, and any suggestion that it
> should use the advance width only would be a change both from its
> original intent and its current wording. I would not support such
> a change for the reasons mentioned above.
>
> If I were to think sizing by the number of ideographic characters,
>> the use of "em" is natural to me. And upright Latin is, for
>> me, ideograph-ized alphabets just like full-width alphabets.
>>
>
> Not quite. A font with proper vertical metrics will not use
> a full em for the advance height of its ASCII characters.


And such fonts will not work with the "ch" definition you're proposing
except for monospace digits. The solution is too restrictive and makes
little sense to me.

[1] https://drafts.csswg.org/css-values/#font-relative-lengths

/koji