Issue #3273317 by clayfreeman, larowlan, tim.plunkett, danflanagan8: Add 'in_preview' variable to block and layout templates

Issue #2546754 by sdstyles, joshi.rohit100: block.html.twig documents a "block" variable, which doesn't exist

Issue #2349625 by mortendk, alexpott, lauriii, Manuel Garcia, saki007ster, emma.maria, DickJohnson: Copy block templates to Classy

Issue #1972122 by akalata, mortendk, JamesLefrère, pakmanlh, ckrina, sun: Remove the DIV tag around block content.

Issue #2328913 by lanchez, lauriii, joelpittet | crowdcg: Move block classes out of preprocess and into templates.

Issue #1053648 by mdrummond, lokapujya, Cottser, Jeff Burnz, Les Lim, Gábor Hojtsy: Convert site elements (site name, slogan, site logo) into blocks.

Issue #2013094 by pwieck, Cottser: Remove unnecessary template_preprocess() reference from all Twig files

Issue #1898034 by Cottser, jenlampton, joelpittet, Shawn DeArmond, idflood, Hydra, artofeclipse, rich.yumul, chrisjlee, gnuget, c4rl, thund3rbox: block.module - Convert PHPTemplate templates to Twig.