[math] math style (minimizing logical height of an equation)

[bkardell]

(Part of #5384 - MathML Core related CSS)

In some cases in mathematical rendering it is desirable to minimize the logical height of an equation. This is generally done by common norms about font-sizing adjustments to parts of the equation, how some operators are drawn, how/where scripts are measured, and how vertical gaps and shifts are applied. Below is an example of the same equation rendered in two common ways, the one on the left applying the common rules to minimize the logical height, the one on the left, not.

For this purpose, we introduce the math-style property with values normal (default) and compact.

MathML supported this via an attribute called displaystyle which was either the string 'true' or the string 'false' ()the rules and the term were derived from The TeXbook). MathML-Core introduces the math-style CSS property and defines how it works for rendering and layout. It maps this attribute's values as a presentational hint to the CSS property for reasons of compatibility with widely deployed content and the existing ecosystem of tools, but strongly recommends authors use CSS.

Tests in https://github.com/web-platform-tests/wpt/tree/master/css/css-fonts/math-script-level-and-math-style

[css-meeting-bot]

The CSS Working Group just discussed math style, and agreed to the following:

• RESOLVED: Add math-style: normal | compact

The full IRC log of that discussion

<fantasai> Topic: math style
<fantasai> github: https://github.com//issues/5387
<fantasai> NeilS: 2 ways of displaying math, one tries to minimize height to fit in lines better
<fantasai> NeilS: one is taller one
<fantasai> NeilS: shifts positions / sizing rules, etc.
<fantasai> NeilS: Designed so you don't have as much gap in lines if you have math in it
<fantasai> NeilS: rules defined in TeXBook
<fantasai> astearns: the switch is true or false for displaystyle?
<fantasai> astearns: I like normal and compact much better
<fantasai> iank_: Makes sense, but seems to also be able to affect the font size?
<fantasai> iank_: that will need to be carefully defined
<fantasai> NeilS: That effect is through the scriptlevel feature
<fantasai> fantasai: Affects the font size, indirectly through another property
<fantasai> iank_: Happy to add, pending scriptlevel discussion
<fantasai> astearns: ok, any concerns to add?
<fantasai> astearns: would it go into display module?
<fantasai> iank_: Goes into MathML Core
<fantasai> RESOLVED: Add math-style: normal | compact

[fred-wang]

@bkardell @fantasai Given #5536 (comment) and that the definition is already present in https://w3c.github.io/mathml-core/#the-math-style-property I guess we can close this, as that was done for #5388 and #5389 ?