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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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