Refactor Getting Started main.css

This was suggested in issue #6320

- Standard properties preferred over vendor prefixes.
- Properties sorted alphabetically.
- Vendor prefixes sorted by length from oldest to newest and longest to shortest to improve readability.
- Added older WebKit gradient syntax to match the age of vendor prefixed box-shadow.
- Fix gradient vendor syntaxes to match their implementation: https://developer.mozilla.org/en-US/docs/Web/CSS/linear-gradient