- not supporting tables in CSS1, even though they were the dominant layout mechanism used at the time.
- `box-sizing: content-box` as original sizing model, which nobody has ever needed.
- `::pseudo-elements`, also used to convince people that having to add extra divs or spans to HTML is a horrible mortal sin to be avoided at all costs.
- the cascade / `!important`, causing endless rule precedence wars.
- `ems` as units relative to the parent instead of the root font size
- making flex-box alignment props the most confusing thing ever
- `display: none` to exclude elements, instead of an `exclude` prop, colliding with the display model
- `position: fixed` not making sense
- `text-overflow: ellipsis` not working in most cases where you'd want it to
I could go on for a while.