Doesn’t give the programmer a lot of freedom to be creative. Using only one language on the web limits the amount features the programmer could create.
For example, just only using HTML limits the web page’s features to use scripts like Javascripts.
In most languages. Primitive data types ( char, int, float, bool, maybe string ) are usually passed by value, but compound data types are passed by reference, so you're not working on a copy but on the original.
