Potentially normative change to target size SC, and other edits

I'm preparing WCAG 2.1 for the next formal publication, scheduled for 
next tomorrow. I routinely do cleanup at this stage to ensure consistency.

In this pass, I came across one issue in the two new Target Size SC 
recently accepted:

https://rawgit.com/w3c/wcag21/master/guidelines/#target-size
https://rawgit.com/w3c/wcag21/master/guidelines/#target-size-(no-exception)

They both linked the term "CSS pixels" to the CSS 2 specification:

https://www.w3.org/TR/CSS2/

That link doesn't really provide value, and we already have a term for 
CSS pixel:

https://rawgit.com/w3c/wcag21/master/guidelines/#dfn-css-pixel

So I changed the link to point to the term in the WCAG 2.1 spec, instead 
of CSS 2.

This is technically a normative change; if anybody objects to it, let me 
know.

Other changes I have made both to recently add SC, and ones currently 
under CfC, which I consider editorial but let me know if you think 
otherwise:

  * Lists in SC changed to definition lists when they have headers;
  * Terms start with a single clause, and any further exposition in
    subsequent paragraphs;
  * Consistent capitalization;
  * Marked everything as "new";
  * Removed stray elements like redundant conformance level markers;
  * Changed some paragraphs to editorial notes when it seemed that was
    the intent;
  * Provide links to Understanding pages (most of them populated just
    with a template);
  * Other invisible edits like making the file we edit match the new
    name of the SC.

Michael