Does anyone know if you can actually write code on Emacs, like HTML/CSS/Javascript?

Do you mean modifying Emacs while it is running?

If you switch to the "* scratch * "[1] buffer and type one of these and after each of these either press «Ctrl-J» or «Alt-x eval-print-last-sexp»:

  (message "Hello, World!")

  (setq cursor-type 'hbar)

  (setq cursor-type 'box)

  (setq indicate-empty-lines t)

  (setq show-trailing-whitespace t)

Is this what you meant?

[1] I have no idea how to get a normal asterisk on HN.