Is it possible to design GUI with HTML+CSS+JavaScript but it will actually run python script?
The popular form of Javascript or ES6 (which you are talking about) is designed to run in browser, so, the limitations are that it can only make calls via browser, i.e. it cannot directly interact with the OS like python's OS module. This means you will need a web-service in your computer that would run a specific python code and return you the responses, this requires a web-service/web-framework, preferably python's like Django, Flask which will run the python script for you because they can make OS calls on the server machine. I do think other non-python web-services are cacpable to do so, but of course, the natural preference would be 'Python-based services'. 
 Sidenote: 
If the case was with Node.js(i.e. the server-side js) and not ES6(client-side browser-run)  you would have an upperhand i.e. you could invoke python scripts on your server because node.js like the python-based web-servers do support os calls.
I've built a very simple assistant app in python which can do very basic tasks like taking notes, reminding you, stopwatch, timer, web scrape for news feeds etc. tkinter seems confusing and looks oldish to me. On the other hand, css js seems much easier to design gui side and way more elegant looking. Is it possible to design a desktop gui app (may be with electron?) using HTML+CSS+JavaScript but it will run my old python codes?
I've been coding for only two months and i suck at it. Please excuse my newbiness.
TLDR: Simply, i want to make the gui side using HTML+CSS+JavaScript to take user input but then it will run python scripts and shows output in the gui app. Is it possible?