This looks like custom syntax to me: x-for="post in posts" You can argue that this is just an HTML attribute, so it is existing syntax. But it appears to

This looks like custom syntax to me: x-for="post in posts" You can argue that this is just an HTML attribute, so it is existing syntax. But it appears to be a way of looping through items in a collection, and as such it is neither HTML nor JavaScript, and therefore it is a custom syntax. It is not checkable at build time if there is no checker available.