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Re: [MathML4] CSS properties for MathML elements and default math font

From: Frédéric Wang <fwang@igalia.com>
Date: Fri, 22 Feb 2019 15:44:54 +0100
To: www-math@w3.org
Message-ID: <9409d0b7-d180-a374-c454-9d872bfb0108@igalia.com>
On 27/07/2016 11:31, Frédéric Wang wrote:
> Hi readers of the Math WG mailing list,
> In a previous message, Murray Sargent indicated that his two main
> complaints about Presentation MathML were "1) lack of an explicit n-ary
> element (for integrals, summations, products, etc.) and 2) lack of
> document level math properties, like default math font". However, MathML
> already has a mechanism to describe such n-ary constructions based on
> the mo/munderover/msubsup elements and the largeop property ; Gecko &
> WebKit rely on them to use the appropriate layout constants. The second
> point is more interesting (especially fonts) so let's open a discussion
> here.
> As you know, stylistic properties in web engines are described using
> CSS. All Web authors are familiar with this mechanism and it is a very
> important feature for them. Users of web browsers can also use custom
> user stylesheets to specify their preferred default style, possibly
> overriding the one of page authors. However, the MathML specification
> does not say much about CSS and so web engines developers have to do
> their own interpretation, leading to incompatibilities between
> implementations. Some parts of the discussion below may be out of the
> scope of the Math WG but I think some hints should be provided in the
> MathML specification anyway.
> First, CSS is much more powerful than the <mstyle> inheritance
> mechanism. That latter element should probably be made compatible with
> CSS or at least should reuse it as much as possible. This is discussed
> in the other thread:
> https://lists.w3.org/Archives/Public/www-math/2016Jul/0024.html
> The specification should probably also explain how many CSS properties
> (e.g. margin, padding, border etc) should be handled for MathML. At the
> moment, Gecko and WebKit essentially ignore these properties in most
> cases. Anyway, the MathML specification is too vague about the exact
> rendering of math elements to open this discussion so I'll postpone it
> for a later thread.
> The default visibility for the mphantom element should be "hidden".
> Although it can be hidden by other mechanisms, using CSS is the most
> natural way to do it and this also has implications in other parts
> outside the rendering module (e.g. the a11y module).
> Many properties should be reset on the <math> element to avoid
> unexpected things. Here is a non-exhaustive list extracted from bug reports:
> * Inheriting the following properties can cause excessive spacing of
> mathematical formulas: text-indent, line-height, word-spacing,
> letter-spacing.
> * In some countries and languages, text is written from right-to-left
> while mathematical formulas are written from left-to-write. Hence it is
> wrong to inherit the direction and we reset that property to
> left-to-write on the <math> tag. Per the MathML specification, authors
> should explicitly use the "dir" attribute on the <math> element if they
> want to force the overall direction of the mathematical formulas.
> Similarly, the writing-mode should probably be set to "horizontal-tb" by
> default.
> * Some properties give poor rendering in math formulas or are confusing
> with the mathvariant style. They should be reset too. This includes at
> least font-style or font-weight.
> Finally, one of the most important issue is the choice of the math font.
> Note that most stylistic math properties are actually included in the
> font itself so it's great that authors can just use the CSS font-family
> to select their preferred fonts (possibly provided as Web fonts), and
> make it consistent with the rest of the page. However, to determine the
> default math fonts, some mechanisms should probably be formalized and
> implemented in web engines:
> 1) Text fonts are not appropriate for math layout. Hence making the
> <math> element inherit the text font is a very bad idea. Ideally, web
> engines should have a mechanism to try and find math fonts in the
> specified CSS font-family lists and to fallback to known math fonts on
> the system.
> 2) One consequence is that we are likely to get different text and math
> fonts and so inconsistent font-size. Instead of making the <math>
> element inherit the font-size, Web engines should probably have a
> mechanism to automatically adjust the font-size.
> 3) Even with this adjustment, the style of the font faces of the text
> and math fonts may be inconsistent. Many text fonts have a math
> companion e.g. STIX / STIX Math, Latin Modern Roman / Latin Modern Math,
> DejaVu Serif / DejaVu Math TeX Gyre etc It would be good to have a
> mechanism to map a font to its math companion (probably this should be
> discussed on the Open Font Format mailing list). Then the font-family
> can be determined using this mapping and this solves all the issues of
> inconsistent style and font-size.
> So to summarize, the idea to determine the default font-family /
> font-size on the <math> element would be: a) find a math font that fits
> best (font-size and fontface style) with the inherited font-family and
> b) adjust the font-size if necessary to match the parent font-size.
> Finally, for the record the user agent stylesheets of WebKit and Gecko
> are available here:
> https://trac.webkit.org/browser/trunk/Source/WebCore/css/mathml.css
> https://dxr.mozilla.org/mozilla-central/source/layout/mathml/mathml.css
> Frédéric
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Frédéric Wang
Received on Friday, 22 February 2019 14:45:29 UTC

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