http://lifeinthegrid.com/simple-css-fixed-header/
http://kb.wpbeaverbuilder.com/collection/7-theme
http://www.creativebloq.com/graphic-design/what-flat-design-3132112
https://www.hongkiat.com/blog/flat-design-resources/
https://www.webdesignerdepot.com
http://chrislord.net/2015/04/24/web-navigation-transitions/
https://css-tricks.com/centering-css-complete-guide/

http://learnlayout.com
https://flexbox.io
http://www.myth.io

Wiki

http://dynalon.github.io/mdwiki/#!index.md
https://ourcodeworld.com/articles/read/782/top-5-best-node-js-based-open-source-self-hosted-web-wiki-knowledgebase-applications