On Mon, Sep 2, 2019 at 7:11 AM Frédéric Wang <fwang@igalia.com> 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.