Agenda+ textPath and SVG unboxing for display:contents

Fantasai has asked for SVG working group review of the parts of the CSS
Display spec that define how `display: contents` would apply to SVG
elements.

https://github.com/w3c/csswg-drafts/issues/2118

That issue is specifically about how to define the different categories of
SVG elements.  If you want the backstory for why the different categories
were used, see https://github.com/w3c/csswg-drafts/issues/540

I'll try to turn my comment there into a pull request before Monday, but
either way I can hopefully help explain what it means.

~Amelia