The size or colour picker is not a real button
Many Shopify themes render variant pickers as a roleless `div` with an `onClick` handler — no `role`, no accessible name, no underlying `select` element. The visible label says "Small Medium Large" but the DOM exposes those words as plain text inside generic divs.
Browser-use agents (Operator, Comet, ChatGPT Atlas, Claude via Playwright MCP) read the accessibility tree before they look at pixels. A roleless div looks like prose. The agent sees "Small Medium Large" as a sentence about a product, not as three options it can pick. It tries to click "Add to cart" without selecting a variant, gets blocked, doesn't know why.
Use a `button` with `role="radio"`, `aria-checked` set per option, and a clear `aria-label` (`Size: M`, `Size: L`). Or a real `select` element. Either works. The Dawn theme (Shopify's default) ships the right pattern out of the box; many premium themes do not.