Adding clarity to CSS examples, esp. splitting redundancies apart.

For example, trying to make it clearer that you wouldn't typically
use "color: black;" and "color: #000000;" in the same rule, but
that those are alternatives to one another.
Fixing demo of black font color, and semantic markup of color list.
Merge pull request #4 from rachelthompson/css-redundancies-gdipgh

Adding clarity to CSS examples, esp. splitting redundancies apart.