(*) A scripting language is a programming language suitable for writing small programs to perform common or one-off tasks.
For example: With a mouse click or when hovering over a button the visitor changes colors on the web page, or texts are rolled out (>>> read more, menu dropdown), pop-ups appear or animations or effects are started. Or after entering numbers, calculations are made...
With only HTML and CSS this is not possible and a web page remains rigid and unchangeable.
Although AJAX is still widely used, modern implementations are making more and more use of JSON.