Re: [webkit-dev] Implementing MathML stylistic attributes in WebKit

[Ryosuke Niwa]

On Fri, Sep 25, 2020 at 12:47 AM Frédéric Wang  wrote:

> On 02/09/2019 16:11, Frédéric Wang wrote:
>
> Hello,
>
> Currently MathML attributes mathvariant, displaystyle and scriptlevel [1]
> [2] are implemented in WebKit using custom "style resolution" and
> "one-glyph rendering" inside the MathML layout code [3] [4] [5]. These
> features involve text rendering and interaction with CSS font-size, so it
> is difficult to implement them properly and completely that way. There are
> known bugs and missing features right now (e.g. mathvariant transform only
> applies to one-character strings, automatic displaystyle/scriptlevel does
> not work with fractions, etc)
>
> Hi,
>
> An update on this, we have been discussing these and new properties with
> the CSSWG:
>
> https://github.com/w3c/csswg-drafts/issues/5384
>
> The math-depth / math-style CSS properties are being implemented in
> Chromium and renamed/exposed/tweaked in Gecko:
>
> https://mathml-refresh.github.io/mathml-core/#the-math-style-property
>
>
> https://mathml-refresh.github.io/mathml-core/#the-math-script-level-property
>
> So it makes sense to implement them in WebKit (it seems we still don't
> have a "intent to" mechanism, but people can follow
> https://bugs.webkit.org/show_bug.cgi?id=195797).
>
> The CSSWG is fine with a new text-transform value for automatic italic but
> using it for other values is controversial (see
> https://github.com/w3c/csswg-drafts/issues/3775). So if we want to
> preserve backward compatibility and fix existing bugs with mathvariant, we
> should probably re-implement it using an internal math-transform property
> (like what Gecko does) which according to our previous discussion is
> nowadays possible in WebKit.
>
Adding the support for new properties sounds good! My concern here is
really only about backwards compatibility.

- R. Niwa
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Re: [webkit-dev] Implementing MathML stylistic attributes in WebKit

[Frédéric Wang]

On 02/09/2019 16:11, Frédéric Wang wrote:
> Hello,
>
> Currently MathML attributes mathvariant, displaystyle and scriptlevel
> [1] [2] are implemented in WebKit using custom "style resolution" and
> "one-glyph rendering" inside the MathML layout code [3] [4] [5]. These
> features involve text rendering and interaction with CSS font-size, so
> it is difficult to implement them properly and completely that way.
> There are known bugs and missing features right now (e.g. mathvariant
> transform only applies to one-character strings, automatic
> displaystyle/scriptlevel does not work with fractions, etc)

Hi,

An update on this, we have been discussing these and new properties with
the CSSWG:

https://github.com/w3c/csswg-drafts/issues/5384

The math-depth / math-style CSS properties are being implemented in
Chromium and renamed/exposed/tweaked in Gecko:

https://mathml-refresh.github.io/mathml-core/#the-math-style-property

https://mathml-refresh.github.io/mathml-core/#the-math-script-level-property

So it makes sense to implement them in WebKit (it seems we still don't
have a "intent to" mechanism, but people can follow
https://bugs.webkit.org/show_bug.cgi?id=195797).

The CSSWG is fine with a new text-transform value for automatic italic
but using it for other values is controversial (see
https://github.com/w3c/csswg-drafts/issues/3775). So if we want to
preserve backward compatibility and fix existing bugs with mathvariant,
we should probably re-implement it using an internal math-transform
property (like what Gecko does) which according to our previous
discussion is nowadays possible in WebKit.

-- 
Frédéric Wang

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Re: [webkit-dev] Implementing MathML stylistic attributes in WebKit

[Frédéric Wang]

On 02/09/2019 22:12, Ryosuke Niwa wrote:
>
> On Mon, Sep 2, 2019 at 7:11 AM Frédéric Wang  > wrote:
>
>
> Currently MathML attributes mathvariant, displaystyle and
> scriptlevel [1] [2] are implemented in WebKit using custom "style
> resolution" and "one-glyph rendering" inside the MathML layout
> code [3] [4] [5]. These features involve text rendering and
> interaction with CSS font-size, so it is difficult to implement
> them properly and completely that way. There are known bugs and
> missing features right now (e.g. mathvariant transform only
> applies to one-character strings, automatic
> displaystyle/scriptlevel does not work with fractions, etc)
>
> Several years ago, Gecko used to do something similar but that was
> causing a lot of problems (dynamic update, assertion failures...).
> At the end, this is now implemented in Gecko more reliably by
> mapping the attributes to internal CSS properties ; [6] is based
> on that. When I tried to do something like this in WebKit three
> years ago, I was only able to rely on proprietary -webkit-*
> extensions exposed to users [7].
>
> So my questions are:
>
> (1) What is WebKit's mechanism to implement such stylistic attributes?
>
>
> It looks like -internal-text-autosizing-status is such a property.
> This is a property the author can never set and we set it internally
> for implementation purposes.
> ComputedStyleExtractor::valueForPropertyInStyle manually skips it so
> it's never exposed anywhere.
>
> One subtle thing I didn't check is whether it would affect the CSS
> parser or not (as in whether it would be considered as a valid CSS
> property or not). There might be some subtle effect there.
>
> -webkit-text-decorations-in-effect is also a CSS property we use
> internally to compute the effective set of text decorations applied to
> text for editing purposes. The author can never set it but it's
> unfortunately exposed via computed style. 
>
> (2) Is it possible to implement internal (i.e. not web-exposed to
> users) CSS properties/values in WebKit?
>
>
> It appears possible today using the technique deployed by
> CSSPropertyInternalTextAutosizingStatus.
>  
>
Thanks for the pointers! I think it is worth trying to (re-)implement
these MathML attributes using internal properties.

-- 
Frédéric Wang

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Re: [webkit-dev] Implementing MathML stylistic attributes in WebKit

[Ryosuke Niwa]

On Mon, Sep 2, 2019 at 7:11 AM Frédéric Wang  wrote:

>
> Currently MathML attributes mathvariant, displaystyle and scriptlevel [1]
> [2] are implemented in WebKit using custom "style resolution" and
> "one-glyph rendering" inside the MathML layout code [3] [4] [5]. These
> features involve text rendering and interaction with CSS font-size, so it
> is difficult to implement them properly and completely that way. There are
> known bugs and missing features right now (e.g. mathvariant transform only
> applies to one-character strings, automatic displaystyle/scriptlevel does
> not work with fractions, etc)
>
> Several years ago, Gecko used to do something similar but that was causing
> a lot of problems (dynamic update, assertion failures...). At the end, this
> is now implemented in Gecko more reliably by mapping the attributes to
> internal CSS properties ; [6] is based on that. When I tried to do
> something like this in WebKit three years ago, I was only able to rely on
> proprietary -webkit-* extensions exposed to users [7].
>
> So my questions are:
>
> (1) What is WebKit's mechanism to implement such stylistic attributes?
>

It looks like -internal-text-autosizing-status is such a property. This is
a property the author can never set and we set it internally for
implementation purposes. ComputedStyleExtractor::valueForPropertyInStyle
manually skips it so it's never exposed anywhere.

One subtle thing I didn't check is whether it would affect the CSS parser
or not (as in whether it would be considered as a valid CSS property or
not). There might be some subtle effect there.

-webkit-text-decorations-in-effect is also a CSS property we use internally
to compute the effective set of text decorations applied to text for
editing purposes. The author can never set it but it's unfortunately
exposed via computed style.

(2) Is it possible to implement internal (i.e. not web-exposed to users)
> CSS properties/values in WebKit?
>

It appears possible today using the technique deployed by
CSSPropertyInternalTextAutosizingStatus.


> (3) Is it ok to add more -webkit-* properties/values or should these
> properties be standardized at the CSS WG instead?
>

N/A

- R. Niwa
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