RFR: JDK-8267709: Investigate differences between HtmlStyle and stylesheet.css

[Jonathan Gibbons]

Please review a new test to track the differences between `HtmlStyle.java` and `stylesheet.css`.

The test passes, because all discrepancies are noted within the test. However, some of the discrepancies may indicate the need for follow-up work, such as adding new items to `HtmlStyle` for documentation purposes, or removing unused code in the main `stylesheet.css` file.

There is also a minor/trivial change to fix a copy-paste error in a comment in `HtmlStyle`.

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Commit messages:
 - polish test
 - update test; explain more discrepancies
 - typo in HtmlStyle
 - typo
 - JDK-8267709: Investigate differences between HtmlStyle and stylesheet.css

Changes: https://git.openjdk.java.net/jdk/pull/4195/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4195&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267709
  Stats: 222 lines in 2 files changed: 221 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4195.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4195/head:pull/4195

PR: https://git.openjdk.java.net/jdk/pull/4195