Why Browser Security UI Isn't Specified

From time to time, working groups (particularly at the W3C) are asked to adopt requirements on browser-presented UI (pixels drawn outside or over-top the page content) in the normative text of a specification. For example, someone might advocate for text like along the lines of "before proceeding to Step 5, conforming User Agents MUST request user consent". Less frequently, advocates will push for the inclusion of specific text to be presented, or in the most extreme cases, specific UI layouts. ...