[css-ruby] Style of intra-annotation white spaces

I notice a problem when I implement the vertical positioning of ruby
annotations. The problem is that, the intra-annotation white spaces cannot
be properly styled, which makes the height of text container look
incorrect. For example, there is a piece of code in the spec:

<ruby>
  <rb>one</rb> <rb>two</rb>
  <rt>1</rt> <rt>2</rt>
</ruby>

After generating anonymous boxes, the frame tree is similar to:

<ruby>
  <pseudo-rbc><rb>one</rb><pseudo-rb> <pseudo-rb><rb>two</rb></pseudo-rbc>
  <pseudo-rtc><rt>1</rt><pseudo-rt> </pseudo-rt><rt>2</rt></psedudo-rtc>
</ruby>

Here, authors cannot specify any style for the <pseudo-rt/>. It won't have
a problem in normal case if UA gives the elements a proper default style.
But in some cases, author may want to specify different style to
annotations, for example, "rt { font-size: 30%; }". Then the boxes for the
intra-annotation white space would be taller than its neighbors.

This problem is relatively less troublesome for <pseudo-rb/>s since authors
can always specify styles to their parents, and let them inherit. But if an
auithor changes the font-size of <rb>, he/she would notice this problem as
well.

I have no idea about any perfect solution. But I suggest that we can make
anonymous intra-annotation white spaces completely collapsed to nothing,
and leave only the anonymous boxes for pairing.

- Xidorn