If the mathml.disabled preference is true, treat <math> and other MathML
elements as generic XML elements.
This patch disables the rendering code of MathML however preserves the
namespace so to reduce the breakage.
Original patch by: Kathy Brade <brade@pearlcrescent.com>
MozReview-Commit-ID: A2f2Q2b4eqR
Unfortunately couldn't add all the debug checks that I'd want, since we can't
assert that is not safe to run script in quite a few places :(
MozReview-Commit-ID: 8m3Wm1WntZs
