what’s the measure without a postfix?
line-height without postfix is based on the current font size (so a bit like em), but with the em postfix, cascading is based on the current font size, so the h1-6 get a line height of 1.7em based on the font size of the main articleselector, so way too small. Without the postfix, element inside the article will have a correct line height, based on their own font size.
Try to insert a really long headline in an article (to have a multi line rendering), with and without this PR to see that the headline are not readable.
See for more details about the unitless value for line-height: https://css-tricks.com/almanac/properties/l/line-height/#article-header-id-0