Custom MathML elements

[fred-wang]

cc @bkardell

Just opening this for the record...

It seems Gecko supports custom elements in the XUL namespace and there is a web components discussion to allow it in any namespace (e.g. SVG too) WICG/webcomponents#634

IIUC the proposal is also to remove the dash requirement for non-HTML namespace, although I'm not exactly sure how that would work with the HTML5 parser.

We already discussed this informally in the MathML CG and agree it would be important to implement MathML elements that are not in the core spec.

[bkardell]

I believe this should move to v2, we aren't going to get there in one step but opening the important discussions of how to align to move forward is the point. Realistically if we have unknown elements we can do most of this part and alignments are mostly important for other steps.

[fred-wang]

from june 1: level 2

[NSoiffer]

Resolved (again?) that this should go to level 2.

Note that the core spec currently labels these as "Unknown MathML element" and provides a few rules about layout if their display value is math or inline-math. I believe no spec change is needed and that this is compatible except if we all extensions of MathML elements such as mfrac, in which case level 2 will need to make changes to that wording (in addition to specifying how these work).