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Quoting the current spec and adding my own comments inline: > emHeightDescent attribute ... > positive numbers indicating that the given baseline is below the bottom of that em square This breaks the following convention otherwise followed by all Ascent/Descent attribute pairs: Ascent = positive numbers indicating a distance going up Descent = positive numbers indicating a distance going down (Thus Ascent+Descent = Height in a sense) I propose that emHeightDescent be brought in line with this convention. > (Zero if the given baseline is the top of that em square.) This was probably meant to say "bottom". To give some context - I am the author of https://github.com/motiz88/canvas-text-metrics-polyfill and actively working on bringing conforming measureText implementations to Chrome and Firefox (Chrome Canary has it, behind a flag and with some issues).