How do we declare a custom attribute in HTML?
Custom attributes are one of the most important new features in HTML5. They play an integral role in semantic development. Custom attributes are designed to store custom data private to web pages or applications for which there are no other attributes. Because custom attributes are valid in HTML5, they get used in all browsers that support HTML5 doctypes.
All attributes in HTML5 on an element starts with "data-" like data-type="best", data-distance="4km", data-identifier="bestinterviewquestion.com".