This SO thread has enough info to at least get started :
http://stackoverflow.com/questions/3767584/html-css-framework-what-should-i-definitely-not-miss?rq=1
If you wanted to start at a higher level start with basic questions.
What is the goal of the framework? Will it be micro for handling very specific cases or like Bootstrap/Foundation and aim to cover most use cases ? Will it include javascript/jQuery ? Which browsers/devices will it support? Will it be HTML5 only ? Which CSS reset will it use? Will it be grid based?
base line typography - font selection, default size adjustments h1-h6 - sans-serif (most likely) p,article - serif (most likely)
a - all states with primary color ol/ul - set indent for nested , remove list-style
UI objects buttons - 2 (at least - primary and default) panel - none rounded- clear padding w / title and footer sections
debatable Hero/Jumbtron
user selects reset and grid