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[csswg-drafts] [css-fonts-4] Managing the scaling of font-size within MathML rendering (#5389)

From: Brian Kardell via GitHub <sysbot+gh@w3.org>
Date: Mon, 03 Aug 2020 20:08:28 +0000
To: public-css-archive@w3.org
Message-ID: <issues.opened-672313673-1596485307-sysbot+gh@w3.org>
bkardell has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-fonts-4] Managing the scaling of font-size within MathML rendering ==
(Part of #5384 - MathML Core related CSS)

Note: We're very interested in CSSWG feedback on this - it has already changed once based on feedback and tests updated, etc - and there has been some additional feedback that perhsaps this should be a single value instead which indicates that perhaps it should consult another (new property).  We're ivery nterested to nail this one down better.

---

Script levels in MathML are about managing the font-size in subexpressions (like scripts and fractions).  [This example](https://mathml-refresh.github.io/mathml-core/#font-size-scriptlevel-example) illustrates and describes some of the concepts/implications.  The method for doing this is not quite as simple as inheriting, so the MathML-Core spec introduces a new `scriptlevel()` function (See example in the [MathML Core spec](https://mathml-refresh.github.io/mathml-core/#the-math-script-level-property) )
 
Depending on context (exponents, fractions, etc) and particulars of rendering style (compact/inline), the font-size of of parts of expressions scale down at different and fairly well established rates - for example, the scaling factor adjustment of a superscript is not the same as that of a root's prescript.  The example below shows several contextual scaling variations.

![rendering of An exponent with an exponent, a root with a prescript and a fraction containing another fraction](https://mathml-refresh.github.io/mathml-core/examples/example-font-size-scriptlevel.png)


The common rules derive from TexBook, but they are subtle and deserve to be exposed for tweaking and extension by authors.  To this end, MathML adds a new `scriptlevel` function for CSS Fonts Module Level 4 which allows authors to express that the value should be automatically derived (default), or explicitly set to a specific level or that it should be the current level + some integer with rules defined in [section 4.5 of the spec](https://mathml-refresh.github.io/mathml-core/#the-math-script-level-property)


Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5389 using your GitHub account


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