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: .
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.
In that case, the validation is performed as users work in the browser.
For example, suppose you specify that a value should be an integer.
For example, you don't want to process a form that's missing critical information.
For example, here's what the page looks like if you forget to enter a course name, if you enter an, and if you enter an invalid date: By default, user input is validated after users submit the page — that is, the validation is performed in server code.
A disadvantage of this approach is that users don't know that they've made an error until after they submit the page.
These pairs let you define rules for both conditions: an error condition and a "normal" (non-error) condition.
It's important to understand that the markup for the error display is always rendered, even if there are no errors.
For example, if a page has an element as a placeholder for an individual field error, even if there is no error.