HTML 5 UP - Makes spiffy HTML5 site templates
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
HTML5 Boilerplate - A rock-solid default for HTML5 awesome.
Purecss - A set of small, responsive CSS modules that you can use in every web project.
HEAD - A free guide to <head> elements
Materialize CSS - A modern responsive front-end framework based on Material Design