reportValidity

In plain words: Custom validation in Lightning Web Components (LWC) allows developers to check user inputs against complex business rules before submission by injecting custom error messages with setCustomValidity() and rendering the native Salesforce red error banner using reportValidity() . While standard attributes like required , minlength , and pattern handle basic constraints, real-world business forms often require dynamic, cross-field validation rules (such as validating email domain formats, custom password criteria, or comparing date ranges). Implementing platform-compliant client-side validation stops bad data before it hits Apex or database triggers.