Why is it that the numerical value for font-size ends with “px” while with font-weight it does not?
Well, to be honest I’ve never asked myself this question but I checked stackoverflow and this is what I found:
Why font-weight is in hundreds?
Apparently it derives from the Linotype typeface classification system (Linotype). Where a 3-number system is used, first numeral describes font weight, second numeral describes font width, third numeral describes position.
There is also an article with some further reading.
Because font weights are not in px, they’re named values (“bold”, “light”, “black” etc) or a number (100, 200, thru 900). It makes no sense for the weight to be in pixels, it’s just an arbitrary name for a certain style of a font family, it bears no relation to any measure (edit: yes there is a classification system, but don’t expect it to actually relate to anything very specific IRL. 400 is visually heavier then 200, medium/book/regular will be heavier than light/extra-light/hairline/whatever (some lines will be thicker, shapes will be adjusted), 700 is heavier than 400, bold/dark/black will be heavier than medium/book/regular (some lines will be thicker, shapes will be adjusted). That’s all it signifies, practically.