Document why mtable sets displaystely to false

[physikerwelt]

In https://www.w3.org/TR/mathml-core/#table-or-matrix-mtable says

The mtable is laid out as an inline-table and sets displaystyle to false.

It's not obvious to me, why this is done. Maybe we want to document that?

See also #125, https://phabricator.wikimedia.org/T418073

[fred-wang]

From MathML 3 https://www.w3.org/TR/MathML3/chapter3.html#presm.mtable

The displaystyle attribute is allowed on the mtable element to set the inherited value of the attribute. If the attribute is not present, the mtable element sets displaystyle to "false" within the table elements. (See Section 3.1.6 Displaystyle and Scriptlevel.)

So if the attribute is not present the UA sheet would set math-style to compact.
If the attribute is present, it's a presentational hint that will set math-style accordingly.

[physikerwelt]

@fred-wang thank you. That explains where it comes from, but not why this is done. In your quote section 3.1.6 is referenced where it says about displaystyle

They also may be set explicitly via the displaystyle and scriptlevel attributes on the mstyle element or the displaystyle attribute of mtable. In all other cases, they are inherited from the node's parent.

So why does mtable does NOT inherit the displaystyle property from its parent and sets it to false.

Some correct test cases:

<mstyle displaystyle=true>
<!-- math-style normal -->
<mtable>
<!-- math-style compact -->

<mstyle displaystyle=true>
<!-- math-style normal -->
<mtable displaystyle=true>
<!-- math-style normal -->

<mstyle displaystyle=true>
<!-- math-style normal -->
<mrow>
<!-- math-style normal -->

For MathML3:

<mstyle displaystyle=true>
<!-- math-style normal -->
<mstyle>
<!-- math-style compact -->

In contrast this is not described in https://www.w3.org/TR/mathml-core/#style-change-mstyle thus in mathml core the correct test case should be

<mstyle displaystyle=true>
<!-- math-style normal -->
<mstyle>
<!-- math-style normal -->

Am I right?

[davidcarlisle]

@physikerwelt

that explains where it comes from, but not why this is done

it's been that way from the start so predates even my involvement in MathML, but I would guess that it was set up that way to follow TeX. Essentially no tex alignments inherit the style, they all use \halign so need to re-enter math each cell and so either force inline (like array) or force display (like align) they never inherit the current style (to do so would involve \mathchoice and setting the content 4 times)

[dginev]

So why does mtable does NOT inherit the displaystyle property from its parent and sets it to false.

I believe the mathematical argument here is that "displaystyle" is used for expressions with a lot of available space to do layout. In contrast, a table cell usually offers a very limited space, and a compact presentation is the more natural default.

To quote MathML Full's framing:

So-called ‘displayed’ formulas, those appearing on a line by themselves, typically make more generous use of vertical space than inline formulas, which should blend into the adjacent text without intruding into neighboring lines.

[davidcarlisle]

Agreed in meeting David to make a PR to the core spec to add a line explaining this style