I think it’s crucial to point out that this tool (while incredibly useful) is only diagnosing your project based on the current page and/or flow (if it’s a single page app). Folks should be careful not to take the results as gospel and start eliminating code without knowing whether it’s actually unused in all scenarios.
Browsers typically exhibit two different behaviors here. Some hide the placeholder text as soon as you focus the field, others hide it only when you start typing. Either one works although, admittedly, I favor the text disappearing when you type rather than when the field receives focus. I can see how that approach might confuse some users, I just prefer it because it ensures you see the placeholder.