HTML, CSS and JavaScript
The three languages a browser understands. HTML says what's on the page, CSS says how it looks, and JavaScript says what it does. Whatever tools an agent uses, these are what finally reach the browser.
Naming the layer narrows the change. "This is a CSS problem" stops an agent rewriting working logic to fix a margin. It also tells you what a screenshot can prove: it shows HTML and CSS, and says nothing about whether JavaScript is working.
Say it to your agent
“This is a CSS problem, so don't touch the JavaScript: the button overflows at 390px wide. Fix it in CSS, use a real <button> rather than a <div> with a click handler, and send me screenshots at 390 and 1440.”