Spec readability improvement

[kof]

What can we do to make it more readable and clear? Please post your suggestions.

[kitten]

I think we should group all marker descriptions by where they occur, e.g.

• Rules
• Style Rules
• Selectors
• At-Rules
• Declarations
• References

Then also we should bump all subtitles from ### to ## and use ### for subgroups so it becomes clearer what section they belong to