Remove some deprecated attributes

[fred-wang]

This proposal is to remove the following attributes:

1. general attribute: other.
2. math attributes: macros, mode.
3. mglyph attributes: fontfamily, index. See also Remove/Deprecate mglyph element #25
4. token elements attributes: fontfamily, fontweight, fontstyle, fontsize, color, background. See also Simplification of the mstyle element #1
5. deprecated namedspace attributes: veryverythinmathspace,
   verythinmathspace, thinmathspace, mediummathspace, thickmathspace,
   verythickmathspace, veryverythickmathspace. https://mathml-refresh.github.io/mathml/chapter3.html#id.3.3.4.2.1 See also Simplification of the mstyle element #1 and Deprecate/Remove namedspaces length values #75

Math WG thread: https://lists.w3.org/Archives/Public/www-math/2016Aug/0008.html

[physikerwelt]

1. Should we suggest using data- instead?

[fred-wang]

These attributes are currently not in MathML Core, so this is just for MathML 4.

[fred-wang]

cc @rwlbuis, @emilio

We agreed on 2019-02-25 that currently deprecated attributes should not go into MathML Core: https://lists.w3.org/Archives/Public/public-mathml4/2019Feb/0048.html

I checked the spec & schema and cannot find any of the attributes listed in #5 (comment)

We still need to:

• Check WPT tests (I doubt there are contained in any test)
• Send intent-to-remove to mozilla/webkit-dev mailing lists (maybe old Math WG too?).
• Remove browser implementation.
• Decide what to do for the full MathML spec (remove them?).
• Add polyfill to https://github.com/mathml-refresh/mathml-polyfills if necessary.

Note: some deprecated attributes might still be used like font or colors (e.g. https://bugzilla.mozilla.org/show_bug.cgi?id=1027354 ). Others have never been implemented or have already been removed without complaints ( e.g. namedspace attributes ).

Also, "mathvariant" is supposed to take precedent over the deprecated attributes "fontweight" and "fontstyle". In term of CSS, that means that mathvariant has to reset font-style/font-weight to avoid weird results. There are some hacks in Gecko/Stylo that can be removed once we get rid of "fontweight" and "fontstyle".

[fred-wang]

From https://lists.w3.org/Archives/Public/public-mathml4/2019Mar/0026.html

1. Remove "other" from MathML full (maybe suggest data-*)
2. Remove macros, mode from MathML full
3. Remove fontfamily, index from MathML full
4. Deprecate fontfamily, fontweight, fontstyle, fontsize, color, background from MathML full
5. Remove named attributes thinmathspace etc

[fred-wang]

Resolution:
#5 (comment)

Specification:
Removal/Deprecation from full, with extra explanations.

Implementation:
WebKit: https://bugs.webkit.org/show_bug.cgi?id=197492
Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1548524
Not implemented in Chromium.

Polyfill:
TBD

Tests:

• Remove WPT tests or check that these values are not supported.

[fred-wang]

These are not in core per #5 (comment) ; @davidcarlisle please check the full spec and do the necessary change, if any.

[fred-wang]

The mode attribute has been removed from Gecko:

https://bugzilla.mozilla.org/show_bug.cgi?id=1573438

(WebKit has never implemented it)

Polyfill:

window.addEventListener("load", function() {
    const namespaceURI = "http://www.w3.org/1998/Math/MathML";
    Array.from(document.body.getElementsByTagNameNS(namespaceURI, "math")).
        forEach(math => {
            if (math.hasAttribute("mode")) {
                var value = math.getAttribute("mode").replace("display", "block");
                math.removeAttribute("mode");
                if (!math.hasAttribute("display"))
                    math.setAttribute("display", value);
            }
        });
});

[fred-wang]

• other, macros, fontfamily, index have never been implemented in browsers
• fontfamily, fontweight, fontstyle, fontsize, color, background was removed in https://phabricator.services.mozilla.com/D43111 and tests were added to mathml/relations/css-styling/attribute-mapping-001.html
• mode was removed in https://phabricator.services.mozilla.com/D41742 but no test were added
• named attributes thinmathspace etc were removed a long time ago from Gecko but I can't find any test

[fred-wang]

* mode was removed in https://phabricator.services.mozilla.com/D41742 but no test were added

• test added for mode: Add MathML reftest to check that the mode attribute has no effect. web-platform-tests/wpt#19113

* named attributes thinmathspace etc were removed a long time ago from Gecko but I can't find any test

We have removed support for namedspaces length values in #75 and have tests for that. So it does not seem necessary to test the legacy namedspaces attributes that allowed to override these values.