The following is an opinionated short list of the specs I find are easy enough for beginner and intermediate designers/developers to wrap their heads around. As I look through more of the specs I'll update this list.
HTML5 - http://www.w3.org/TR/html5/sections.html#sections
CSS Selectors Level 3 - http://www.w3.org/TR/css3-selectors/
CSS Selectors Level 4 - http://www.w3.org/TR/selectors4/
CSS Counter Styles Level 3 - http://www.w3.org/TR/css-counter-styles-3/
Compositing and Blending Level 1 - http://www.w3.org/TR/compositing-1/
CSS Masking Module Level 1 - http://www.w3.org/TR/css-masking-1/
CSS Transforms Module Level 1 - http://www.w3.org/TR/css-transforms-1/
Geofencing API - http://www.w3.org/TR/geofencing/If you think there is a spec that I missed suggest it in the comments section and I'll take a look.