This directory is just a placeholder for an eventual CSS Block Layout Level 3 module.
In the meantime, please refer to CSS2 chapters 8, 9, 10.
  https://www.w3.org/TR/CSS2/

For anyone taking up this project, there existed a first draft by Bert Bos, see
  https://www.w3.org/TR/2018/WD-css3-box-20180731/
It was not kept in sync with fixes to CSS2, but has some useful descriptions and diagrams.
That draft's image sources have been copied into this directory in case they would be useful.

