Speaking of ancient CSS features that looked overwhelmingly promising but never caught up as successfully as they deserved, it is probably worth mentioning User Styles. CSS was initially meant as a "dialogue" among User Agent, User, and content Author, with styles as a tool to express individual preferences. User agent sets some "browser defaults", Author sets their styles and User can participate with their own sets of preferences that can be "weak" like those defaults or even stronger than author's. It is the (or at least mayor part of) "THE 'C' for 'Cascade' in CSS".
Browsers to this days are required by specs to give user a way to participate in the cascade, but it never wasn't easy or even pleasant experience - it mostly involved editing some magic file and restarting browser (for "native" user styles) or using some extension, what feels like a poor excuse to not support it natively and mostly is implemented an a way that does not comply with specs anyway - because it mostly just spams author origin level and does not create user origin level. (I miss the old days when web was young and "userstyles" and "userscripts" seemed like natural development of tech what every user will use daily.)
Browsers to this days are required by specs to give user a way to participate in the cascade, but it never wasn't easy or even pleasant experience - it mostly involved editing some magic file and restarting browser (for "native" user styles) or using some extension, what feels like a poor excuse to not support it natively and mostly is implemented an a way that does not comply with specs anyway - because it mostly just spams author origin level and does not create user origin level. (I miss the old days when web was young and "userstyles" and "userscripts" seemed like natural development of tech what every user will use daily.)