The Design style guide should display the applicable validation states (error, warning, success, notice( ?)...) of input components.
At the moment, only limited validation guidelines are provided, and several divergent approaches can be found in production (red border + red text, red border only, with or without inline validation message, persistent red border on focus or not), so it's hard to infer any style patterns. Some live examples of how validated components are currently styled:
Link input field (VE):
Input/combobox (VE):
Button/dropdown (VE):
Lookup (Structured data, Commons):
Point in time selector (Structured data, Commons):