What is the mask in an input element in HTML?

In HTML, the mask attribute is used in the <input> element to specify a pattern that the input value must match. This pattern can be used to ensure that the user enters data in a specific format, such as a phone number, date, or credit card number.

The mask attribute uses a special syntax to define the pattern. For example, a mask for a phone number might look like this: (999) 999-9999. The 9 character in the pattern represents a digit, while the parentheses and hyphen are literal characters that will appear in the input. When the user types into the input, the browser will only allow them to enter characters that match the pattern.

Note that the mask attribute is not a standard HTML attribute, but rather a feature of some JavaScript libraries or frameworks like Input Mask, jQuery Mask Plugin, or VanillaMasker. Additionally, the mask attribute is not supported by all browsers, so it is important to provide alternative validation methods for users who may be using older or unsupported browsers.