What you're building
Add one @media query that changes the layout on smaller screens. This is genuinely the 80/20 of "make it work on phones" — one rule, enormous payoff.
How the editor knows you're done
The code contains @media somewhere in the CSS.
A starting point
Add this to the bottom of your CSS panel:
/* Stack the layout vertically on phones */
@media (max-width: 768px) {
.container {
flex-direction: column;
padding: 12px;
}
h1 { font-size: 1.5rem; }
}Now drag the corner of the preview to make it narrow — or open the preview in a new window and resize it. You'll see the layout reflow at 768px wide.