[MathML4] Removal of the menclose notation "radical"

Hi everybody,

Continuing on proposal for MathML4, there are two equivalent notations
for square roots:

* <msqrt> child1 child2 ... child3 </msqrt>
* <menclose notation="radical"> child1 child2 ... child3 </menclose>

The latter does not bring anything new but duplicate code or
inconsistencies in implementation. In WebKit, the msqrt and mroot
elements share implementation and menclose@radical was implemented by
creating anonymous nodes. In the past, this kind of implementation has
led to rendering, performance and design/security issues. During phase 1
of our refactoring [1], we thus removed support for menclose@radical. In
Gecko, the msqrt and menclose elements share implementation but then
mroot has duplicate logic, which is bad and Gecko should probably be
aligned on WebKit here.

So we would like menclose@radical to be removed in future versions of
MathML (it is optional in MathML3 anyway). We expect this removal to be
safe for the users given that (except in examples and tests) msqrt is
always preferred over menclose@radical in math documents. The only
rationale for menclose@radical would be to write overlapping notations
(e.g. <menclose notation="radical circle horizontalstrike">) but again
we are not aware of any concrete use cases for that and it's always
possible to nest <msqrt> and <menclose> to get similar rendering without
overlapping notations.

Frédéric, for the Igalia Web Platform team

[1] https://trac.webkit.org/wiki/MathML/Early_2016_Refactoring