[css-ruby] Apply consistent formating throughout the source code

* tabs for indentations, spaces for alignment
* semantic line breaks https://rhodesmill.org/brandon/2012/one-sentence-per-line/
* Indent the entire spec one level except for headings
* Line break after opening heading tag
* etc (see https://lists.w3.org/Archives/Public/spec-prod/2018OctDec/0011.html
  for more on this style)

Much of the spec was already like that, but not everything. Now it's all
consistent.

This commit makes no change the text of the spec.

Author: frivoal