Issue #2505701 by pwolanin, YesCT, akalata: Document SafeMarkup::set and Use htmlspecialchars() directly in Attribute() so we don't bloat the list of safe strings

Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #2227869 by joelpittet: Remove magic printing from Attribute class to make Twig 'without' filter work for attributes.

Issue #2212309 by joelpittet: Drupal\Core\Template and twig.engine docs, coding standards, and unused code cleanup.

Issue #2100329 by cosmicdreams: Use \Drupal instead of Drupal to make IDEs and static code analyse tools happy.

Issue #2006282 by joelpittet: Refactor Attribute classes - Cleanup, Security, and Readability and minor performance.

Issue #2083941 by dawehner: Fixed \Drupal\Core\Theme\Attribute->value() is named wrong and does not work.

- Patch #1290694 by hefox, tlattimore, c4rl, chx, tim.plunkett, tstoeckler, msonnabaum: provide consistency for attributes and classes arrays provided by template_preprocess().