[css-ruby] Conflict with HTML5 spec

I just notice a conflict between the CSS Ruby spec and the sections related
to ruby in the HTML5 spec.

The HTML5 spec of both WHATWG and W3C version say that improperly-contained
inner ruby elements should represent their children (Including that <rp>
should not be hidden). [1][2]

However, the CSS Ruby specifies the display value unconditionally to those
tags in the default stylesheet [3], and requires generating proper parents
for those elements [4].

I guess the easiest way to fix this conflict is changing the default
stylesheet to meet the HTML5 spec.

[1] https://html.spec.whatwg.org/multipage/semantics.html#the-rt-element
[2] http://www.w3.org/TR/html5/text-level-semantics.html#the-rt-element
[3] http://dev.w3.org/csswg/css-ruby-1/#default-ua-ruby
[4] http://dev.w3.org/csswg/css-ruby-1/#box-fixup

- Xidorn