Basic level:

• enclose code in ``` tags instead of pre, code etc.
• code style: spacing, indentation etc.

Intermediate level:

• unify the way code reads parameters and reports output. A lot of code has comments instead of reading input - we could, for example, wrap such code in a function which accepts a certain list of parameters, so that we don't need a specific input-handling code, and still the code is complete and can be used as a library method.
• use the same language for all generic code (i.e. code which doesn't highlight some specific feature of a specific language). Some articles have the same code in C++ and Python, and the difference is purely syntax.

Advanced level:

• make sure all code compiles and is covered by unit tests :-) We could use Jupyter to embed executable code in our articles.