There is a bias against the value of design in FOSS, the dismissive attitude described does exist, but I don't think that bias is gender based. The argument draws from outdated principles about gender division where "female" = "artistic/emotional/right-brain" and "male" = "scientific/rational/left-brain", concluding that HTML and CSS being "layout" makes them "design" and therefore more "feminine."

But HTML and CSS aren't art, nor is layout a "creative" activity. HTML and CSS are configuration. They're no more feminine than JSON or a Lua table. I don't think there's ever been a time where, even if the gender bias described existed, it was ever applied in the way described.