If it’s of use to anyone else, I have just discovered that the font netflix uses for Japanese is MS Gothic as a generic placeholder for TTS’ proportionalSansSerif. I altered the font-family on the css Lea provided and while it’s not the same as the default font, at least now there’s no more weight changes between kanji and kana. c:
(Source: Netflix’s Japanese Timed Text Style Guide)
I also bumped into this, if anyone finds it interesting:
Netflix’s Tech Blog: Implementing Japanese Subtitles on Netflix.
It’s not too useful for a regular end user, but more of a slightly off topic curiosity if anyone’s interested in seeing their inner workings. c: