Ensure elements do not contain duplicate attributes
no. 017
Make sure attributes are not repeated in any given HTML element
Browsers and other user agents, including assistive technologies, can accurately interpret HTML content as long as attributes within elements are used according to the HTML specification. In other words, as long as each HTML attribute is only used once in any given element, as potential problems may easily arise, when developers maintain different and potentially conflicting attributes values for a single HTML element. The goal is to avoid duplication errors that might cause assistive technologies to inaccurately interpret the markup and lead to misleading or false information conveyed to users (for example, overwriting the first attribute value by a second that potentially conveys the wrong information, or vice versa).
To do this, simply make sure the parsing of the document runs smoothly by verifying that every attribute in any given element is only used once. Check this manually, or rely on a validation service such as the W3C Validator to make sure the markup does not contain this type of errors. When HTML documents are free of such duplication errors, the interpretation or parsing by user agents, including assistive technologies such as screen [...]