An '''HTML attribute''' is a modifier of an [[HTML element]]. They modifyare theusually actionadded ofto a [[HTML tag|HTML start tag]], which along with ''content'' and ''end tag'' comprises themost HTML elementelements. Particular attributes are only supported by particular tagselement classes; if they are added to tagselements that do not support them they will not function. Some attributes are ''required attributes'', needed by particular tagselement classes for them to function correctly; while in other cases they are ''optional attributes'', only needed when something other than the default formatting is required. ''Standard attributes'' are supported by a large number of tagselement classes; they include ''event attributes'', which trigger elements to specify scripts to be run under specific circumstances.
