What your team member is describing is the CSS attribute selector. If anything, his/her way of targeting the <form>
element would be less backwards compatible as the ability to target elements based on attributes was added in CSS 2.1 while your way of doing it has been around since the beginning. Although less compatible, the market share of browsers that do not support CSS 2.1 is very minimal, and shrinking every day.
See here for more info: http://www.w3.org/TR/CSS2/selector.html#attribute-selectors