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

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

Frédéric Wang-2
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

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:

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,

* 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

* 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: