When users fill up forms while making a purchase or booking a flight ticket, they want to stay informed about what's happening in the system.

To make the error inputs easy to notice and understand:

• Indicate error inputs using a prominent, contrasting color (usually red) and a relevant icon. 
• Politely describe what happened and what users should do to fix the problem. 
• Apply real-time, inline validation rather than submit-time modals. The latter breaks the flow and distracts users. Errors that appear immediately allow users to correct their answers before moving on.^[1]