Sometimes you have to validate information that doesn't come directly from an HTML form.
A typical example is a page where a value is passed in a query string, as in the following example: source, including form-field values, query-string values, and cookie values.
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However, the test will be performed in server code.
You can control how validation errors are displayed by defining CSS classes that have the following reserved names: .
Performing validation in server code is a security measure, in case users bypass client-based validation.
In this example, the test for a valid date won't work in client code.
If a form is long or complex, reporting errors only after the page is submitted can be inconvenient to the user.