Make the form understandable first
A required field tells the browser that a value cannot be empty. It does not tell the user why the field matters or what a useful answer looks like. Before adding complex rules, give each field a clear label and explain any unusual requirements close to the input.
MDN describes client-side validation as a way to catch mistakes before a round trip to the server. Built-in HTML controls can check required values, lengths, ranges and common formats. Those checks improve the experience, but MDN also warns that they are not an exhaustive security measure.
Repeat the rules on the server
A person can bypass the form and send a request directly. The server therefore needs to validate submitted data even when the browser already appears strict. A dropdown, for example, should accept only the allowed values on the server; the visible options do not constrain a manually constructed request.
OWASP recommends defining expected fields, types, lengths and allowed choices. Reject unexpected structures before they reach business logic. For collections, decide how many items are allowed and validate each item. Keep limits reasonable for the task instead of accepting an arbitrarily large payload because the interface normally sends a small one.
Check whether the value makes sense
Syntactic validation asks whether a value has the expected shape. Semantic validation asks whether it makes sense for the operation. OWASP uses consistency between related fields as part of this distinction. An end date can be a valid date while still being earlier than the start date.
Do not confuse validation with every other defense. OWASP separates it from parameterized database queries, context-aware output encoding and authorization. A valid account identifier does not prove the caller owns that account. A string passing a length check is not automatically safe to insert into HTML or a database query.
Design the failure state
When a field is wrong, explain what the user can change and preserve appropriate input. Test several errors together rather than only one missing field at a time. Make sure keyboard users can find the error and that the correction does not silently discard the rest of the form.
Finally, test the successful submission all the way to its destination. A green banner should mean the application accepted the action, not merely that a button was pressed. Good validation protects the system while making the ordinary person's next step clearer.