Questions · Buy
Why is a cart that needs JavaScript medium, not critical?
Because agents that drive a full browser can often get past it, so it blocks some agent traffic rather than all of it. The stalls rated critical block every agent, including the ones running a real browser.
Why it happens
A form that posts, or a link that navigates, is an action any client can take. A button whose only behaviour is attached at runtime is an action only a client running that runtime can take. The difference is invisible on screen and total in effect.
This stall sits at the narrowest part of the journey, which is what makes it costly. Everything upstream succeeded: the product was found, read, and selected.
This comes up at the Buy stage. The full treatment, including how to reproduce it and what to change, is on Add to cart needs JavaScript the agent cannot run.
Related questions
Run a free scan and find out which of these your store actually hits.