JSX is easy to summarize as: HTML where everything in curly brackets is a JS expression that gets evaluated. There are some additions (prop spreading) and restrictions (curly brackets apply to whole attribute values and element bodies only), but they're very simple.
And custom event handling attributes, camel casing, special cases like htmlFor and className, non-standard attributes being ignored except for custom elements, the style attribute, the special case of checked/selected properties vs attrs for form elements, key and ref properties. Plus all the semantics of component updates, memoizing and so on.
This is already far more to learn than the handful of simple control structures Svelte introduces.
