[css-inline] Define line-height: normal a bit more precisely

[fantasai]

http://meyerweb.com/eric/thoughts/2008/05/06/line-height-abnormal/

(Comments on what 'line-height: normal' means in various browsers welcome.)

[fantasai]

Minutes of the 2017 Tokyo F2F discussion
Minutes of 2017 Paris F2F discussion
Minutes of 2017 Sept 27 telecon
Minutes of 2017 SF F2F discussion = resolutions on definitions of line-height calculations

Meta-bug on line-height issues
Marking this closed based on above discussions and the consequent edits.