Simplification of the mstyle element

[fred-wang]

This proposal is to simplify mstyle so that it only supports the following attributes:

• displaystyle
• dir
• mathsize
• mathbackground
• mathcolor
• mathvariant
• scriptlevel

And maybe:

• scriptsizemultiplier
• scriptminsize

From the Math WG mailing list:
https://lists.w3.org/Archives/Public/www-math/2016Jul/0024.html

From the appendix of the original "MathML in HTML5" document:
"The mstyle element duplicates the CSS inheritance mechanism in a CSS-incompatible way and has many exceptions to workaround the fact that attribute names may be used for different purposes. In practice, many of its attributes are not useful and never used. It is a burden for implementers since they essentially have to reimplement a specific “attribute” inheritance mechanism to support the general case even if the most prominent attributes have obvious mapping to CSS. It is also a performance issue to perform the rendering and keep it up-to-date since the rendering on any node may depend on its mstyle ancestors. In this document, it instead suggested to keep only attributes that are compatible with CSS and five new CSS properties are introduced in section 2.3.1. These attributes actually correspond to what is used in practice. "

edit:
This is the proposal for CSS attributes that cannot be mapped to style yet: https://github.com/mathml-refresh/mathml-css-proposals#mathml-css-proposals

[physikerwelt]

This might be a stupid question, but why do we need

• dir -> direction
• mathsize -> font-size
• mathbackground -> background-color
• mathcolor -> color
  can that not implemented using the style attribute?

[fred-wang]

Yes, they can be (and are) implemented using CSS style.
Whether we want to remove/deprecate these MathML attributes (and hence the whole mstyle element) is another question, but probably they have already been used a lot in existing documents. This is not really the case of attributes not mentioned in #1 (comment) , I think in a previous discussion on Math WG only one instance of another attribute (linethickness) was used in the MathML 3 test suite.

[fred-wang]

Incidentally, for attributes without CSS equivalent, there is https://github.com/mathml-refresh/mathml-css-proposals#mathml-css-proposals

@physikerwelt I think in general it would be good to gather usage metrics of elements/attributes that are proposed for deprecation/removal. Maybe you can do that for wikipedia.

[davidcarlisle]

This might be a stupid question, but why do we need

* dir -> direction

* mathsize -> font-size

* mathbackground -> background-color

* mathcolor -> color
  can that not implemented using the style attribute?

html also has a dir attribute so this isn't so strange I think?

Initially the model was to preserve as far as possible that as far as possible the notion that css should be used to encode potentially reader-customizable style, and that explicit attributes in the source should be used to encode author-specified meaning.

So a clearer example would be <mi mathvariant="sans-serif" to chose a sans math alphabet as opposed to using CSS to chose the overall font style (which may be sans serif)

Of course in the end mathvariant got defined as a shift of codepoint rather than a change of font, but it was still a motivating example here.

So at the time, almost all publications were monochrome but the thought was that you may want to use colour just as fonts have been traditionally used, to have distinguished mathematical variables x mathcolor="red" and x mathcolor="blue" that are separate from a reader choice to style the whole expression in brown to match a website style...

We can decide not to keep this (and we could potentially have different decision for the mathml-core and full mathml but if you are asking why mathml3 (and 2 and 1) are this way, that is basically why.

[dani31415]

Why not indentalign -> text-align ?

[physikerwelt]

discussed in the telco on 2019-02-25:

• @NSoiffer does not think one can get rid of mstyle entirely?
  ** @fred-wang "in theory one could get rid of mstyle and use style of mrow"
• can one get rid of matvariant?
• patrick ion asks if there are glyphs that are only accessible via mstyle?

[fred-wang]

Why not indentalign -> text-align ?

I don't think indentalign is in MathML Core. Also we should keep in mind (2) #30 (comment) applies here.

[fred-wang]

This is how I understand the current status:
(1) Do we keep the <mstyle> element? It could just be <mrow> + CSS styles but we need (a) to define more CSS properties / remove some attributes and (b) take into account non-browser implementation and backward compatibility with existing usage #55.
(2) Do we reduce the existing set of properties (suggestion: #1 (comment)).
(3) Do we just map properties to CSS instead of defining our own inheritance?
(4) Do we move new proposals https://github.com/mathml-refresh/mathml-css-proposals to CSS -drafts?. Per https://lists.w3.org/Archives/Public/public-mathml4/2019Mar/0007.html for mathvariant it seems fine to extend CSS text-transform and displaystyle is not really controversial. I hope we can decide about scriptlevel-related properties in the next meeting.

[dani31415]

I think we should make MathML core as much backward compatibility as possible with existing formulas. With this idea we should implement the <mstyle>.

At the same time I would deprecate <mstyle> in the sense that we recommend to use the alternative version of <mrow> with CSS. In addition, in MathML 4, I would deprecate <mstyle> as well (with the same meaning of a recommendation of not to use it).