C validating user input
This means that validation needs to be carried out server-side as well.Where possible, users should be able to check their input and correct it if necessary.If your web browser supports HTML5, it will not allow you to submit the form without entering text into the input field.Instead, it will display a message that is generated by the web browser itself.Developers are often provided with a large amount of security advice, and it is not always clear what to do, how to do it, and how important it is.Especially considering that security advice has changed over time, it can be confusing.In some situations, such as validating custom controls or supporting legacy browsers, additional scripting may be necessary to validate user input.Custom validation needs to notify users in an accessible way as described in the User Notifications part of this tutorial.
In the example below, the element specifies a particular format that matches car license plate (registration) numbers in Germany.
Also, the attribute can be added to form controls, to programmatically indicate that they are required.
Most current web browsers support this attribute and will communicate missing required input to the user, using standard web browser dialog mechanisms.
The required pattern consists of one to three letters (for the city where the car is registered), followed by a space, two to four random letters, another space, then one to four random numbers.
Validation should aim to be as accommodating as possible of different forms of input for particular data types.