Issue #2118663 by cdog, nod_: Remove the Drupal.checkWidthBreakpoint method from drupal.js; replace with window.matchMedia where appropriate

Issue #2566771 by Cottser, mortendk, jstoller, LewisNyman, mdrummond, Gints Ērglis, sqndr, Chernous_dn, Manjit.Singh, axe312, emma.maria, davidhernandez, lauriii, jaxxed, akalata: [Voltron patch] Move all remaining *.theme.css to Classy

Issue #2554219 by nod_, larowlan, tim.plunkett, regilero, droplet, japerry, samuel.mortenson, Pere Orga, effulgentsia, benjy, Gábor Hojtsy, greggles, Wim Leers, David_Rothstein, pwolanin, neclimdul, EclipseGc, znerol: Port Cross-site Scripting - Ajax system fixes from SA-CORE-2015-003 to Drupal 8

Issue #977844 by Wim Leers: Remove the 'every_page' option for CSS/JS assets: it is confusing, even damaging

Revert "Revert "Issue #2510104 by pwolanin, nod_, Fabianx, Wim Leers, droplet, Pere Orga: Convert drupalSettings from JavaScript to JSON, to allow for CSP in the future""

This reverts commit e8e883f9.

Revert "Issue #2510104 by pwolanin, nod_, Fabianx, Wim Leers, droplet, Pere Orga: Convert drupalSettings from JavaScript to JSON, to allow for CSP in the future"

This reverts commit 94615a13.

Issue #2510104 by pwolanin, nod_, Fabianx, Wim Leers, droplet, Pere Orga: Convert drupalSettings from JavaScript to JSON, to allow for CSP in the future

Issue #1835016 by googletorp, nod_, KarenS, Sharique, Nitesh Sethia, mpdonadio: Polyfill date input type

Issue #2472177 by Dom., nod_, drubb, mgifford: Collapsible fieldset have duplicated and wrong aria-expanded

Issue #2485579 by ram4nd, droplet, nod_, MustangGB, evilfurryone, iMiksu: Update JS library picturefill to 2.3.1

Issue #2419897 by penyaskito, Wim Leers, webflo: Javascript translations are loaded in the wrong order due to missing dependencies

Issue #2260061 by Jelle_S, attiks, Wim Leers, mdrummond, mgifford, holist: Responsive image module does not support sizes/picture polyfill 2.2

Broke testing UI. Reverted.

This reverts commit dc5ef576.

Issue #784626 by Wim Leers, Manuel Garcia, jcisio, joelpittet, Bevan, fcaspani, dcmouyard: Default all JS to the footer, allow asset libraries to force their JS to the header

Issue #2368797 by Wim Leers, dawehner, rteijeiro: Optimize ajaxPageState to keep Drupal 8 sites fast on high-latency networks, prevent CSS/JS aggregation from taking down sites and use HTTP GET for AJAX requests